“Those, who denounce fear, assail the law; and if the law, plainly also God, who gave the law…. If, then, according to hypothesis, they abolish the law; then, by necessary consequence, each one who is led by lust, courting pleasure, must neglect what is right and despise the Deity, and fearlessly indulge in impiety and injustice together, having dashed away from the truth.
Yea, say they, fear is an irrational aberration, and perturbation of mind. What do you say? And how can this definition be any longer maintained, seeing the commandment is given me by the Word?
….Fear is not then irrational. It is therefore rational. How could it be otherwise, exhorting as it does, You shall not kill, You shall not commit adultery, You shall not steal, Than shall not bear false witness? But if they will quibble about the names, let the philosophers term the fear of the law, cautious fear, (εὐλάβεια) which is a shunning (ἔκκλισις) agreeable to reason…. Cautious fear (εὐλάβεια) is therefore shown to be reasonable, being the shunning of what hurts; from which arises repentance for previous sins. For the fear of the Lord is the beginning of wisdom; good understanding is to all that do it. He calls wisdom a doing, which is the fear of the Lord paving the way for wisdom. But if the law produces fear, the knowledge of the law is the beginning of wisdom; and a man is not wise without law. Therefore those who reject the law are unwise; and in consequence they are reckoned godless (ἄθεοι). Now instruction is the beginning of wisdom. But the ungodly despise wisdom and instruction, Proverbs 1:7 says the Scripture” (Saint Clement of Alexandria, The Stomata II.7)
The fear of the Lord helps us to overcome sins to which we might fall through ordinary worldly fear. It replace and thus drive our human fears, thus leading to fortitude. The philosopher-historian Rusell Kirk puts it this way:
“Meek before Jehovah, Moses had no fear of Pharaoh; but these doctors of the schools, much at ease in Zion, were timid in the presence of a traffic policeman. Although convinced that God is too indulgent to punish much of anything, they were given to trembling before Caesar. Christian love is the willingness to sacrifice oneself; yet I would not have counted upon these gentlemen to adventure anything of consequence for my sake, nor even for those with greater claims upon them….
What raises up heroes and martyrs is the fear of God. Beside the terror of God’s judgment, the atrocities of the totalist tyrant are pinpricks. A God-intoxicated man, knowing that divine love and divine wrath are but different aspects of a unity, is sustained against the worst this world can do to him; while the goodnatured unambitious man, lacking religion, fearing no ultimate judgment, denying that he is made for eternity, has in him no iron to maintain order and justice and freedom” .
If “fear” has become a word associated with the purely negative connotation of “worldly fear,” it is largely because we have lost the true meaning of holy Fear. Worldly fear can lead to cowardice, it is true, and an excess of servile fear which never progresses through initial fear to filial fear can lead to scrupulosity or to spiritual paralysis. However, it is through Holy fear, including through the merely servile variety, that we can develop a sort of courage ; indeed as Ven. Louis of Grenada notes, each of the other gifts builds on fear of the Lord, beginning with piety and knowledge and progressing next to fortitude before at last ascending through counsel, understanding, and finally to wisdom. Saint Augustine puts it  more succinctly still in when he says that the martyrs can say, “I do not fear [man], because I fear [the Lord].”
There are two related objections which come to my mind, which are both related and both rooted in a poor understanding of the synthesis of Old and New Convenants, especially as found in Old and New Testaments. The first objection is that this holy fear is rooted too much in a concept of God as a wrathful deity—which is often how He appears to be portrayed in the Old Testament. But (so the misguided interpretation goes) in the New Testament, we see that God is a God of Love, and this God supplants (or replaces!) the wrathful God of the Old Testament. The second objection builds off of this first objection, and attempts to back it up by quoting a single passage taken as summary of the whole:
“Beloved, let us love one another; for love is of God, and he who loves is born of God and knows God. He who does not love does not know God; for God is love….So we know and believe the love God has for us. God is love, and he who abides in love abides in God, and God abides in him. In this is love perfected with us, that we may have confidence for the day of judgment, because as he is so are we in this world. There is no fear in love, but perfect love casts out fear. For fear has to do with punishment, and he who fears is not perfected in love” (1 John 4:7-8, 16-18).
It has become a popular opinion today, especially among Christians of a “progressive” bent, to quote passages like this  and then conclude that we have moved beyond fear. “We have love, so why do we need fear?”
Yet, as Burk notes, a crucial step in learning to love what we ought to love is learning to hate what we ought to hate. This is the meaning of Christ’s own admonition that “If anyone comes to me without hating his father and mother, wife and children, brothers and sisters, and even his own life, he cannot be my disciple. Whoever does not carry his own cross and come after me cannot be my disciple” (Luke 14:25-27). I would add that the first step in this process of learning to love (and yes, learning what might be called “righteous hate”) is learning what to properly fear, that is, what to rightly dread.
This includes above all sin, which separates us from God. It also must include those things which tempt us to sin, including what Mnsr. Charles Pope has referred to as “the sin of human respect,” which is “that sin wherein we fear man more than God; where we more concerned with what people think of us and what we do, than what God thinks. This is an unholy fear, a sinful fear which is at the root of a lot of sins we commit as well as of many sins of omission.” When we fear men rather than God, we put the desires of men above the designs of God, which is not wisdom but rather is folly. When we fear the power of the world—this is a form of worldly fear, human fear—more than the punishments or even the pleasure of God, then it becomes difficult to follow Him faithfully.
Perhaps it is for this reason that Saint Paul counsels us to “work out your salvation with fear and trembling” (Philippians 2:12). It is not counsel to be afraid, in the worldly sense, but rather to be aware that God is present, to take heed lest we slip into immoral and ungodly patterns of living; it is an “alarm,” in Pope Francis’ words, against settling oneself into evil; indeed, fear of God is an alarm even against simply spiritual complacency, whereby we cease to desire that our souls progress closer to God. In such complacency, we say to ourselves that we are now good enough, and need no further help from grace, or need cooperate no more with it. We say to God, in effect, “this far, and not a step more”; in contrast, He says to us that that we must take up our crosses daily in order to follow Him (Luke 9:23). It is for this reason that the author of the Epistle of Barnabas tells us that “Fear and patience, then, are helpers of our faith; and long-suffering and continence are things which fight on our side.”
What, therefore, should we make of the words of Saint John when he tells us that “There is no fear in love, but perfect love casts out fear”? Saint Augustine appears to take especial interest in this question, in that he returns to it several times in his sermons and writings: for example, we see him address this point in his Tractates on the Gospel of John (Tractates 43 and 85), in his homily on the First Epistle of John, and in his letters.
The solution which he proposes is the previously mentioned distinction between chaste and unchaste fear, and with it the distinction between hating sin and merely avoiding it. In his letter to Anastasius (ca. AD 412-413), he notes that one can avoid sin because one fears the consequences of sin—that is, a man can refrain from committing a sin in order to avoid the punishment which that sin will bring. This is, of course, true in regards to crimes against God, but also in regard to crimes punishable by the state.
On the other hand, we might avoid sins for the reason that we actually hate them, that is, because they become unthinkable to us. They may be unthinkable on the face of the thing, for example the idea of murdering my wife or daughter is absolutely unthinkable to me, both because I love God and know this to be contrary to His commandments (see John 14:15, 23), but also because I love my wife and my daughter and could not stand the thought of doing such a horrible thing. This is, incidentally, what is meant by St Paul when he says that love fulfills the law (Romans 13:8-10, Galatians 5:14): “thou shalt not kill” seems less like a burdening law and more like an obvious afterthought when you love the person whom you are not killing.
Where chaste fear comes into play is that there are some “laws” or “commandments” which are less obvious on their face . That we should not commit rape or adultery ought to be obvious as conditions of loving a person and doing so faithfully. Yet, fornication, pornography, and “sexting” are amongst the things which are (implicitly or explicitly) proscribed by the Law. They are ultimately also not consistent with loving God and neighbor, yet this inconsistency is less obvious than is the case with rape; to the modern mindset at least, it seems at times as if the only reason to not engage in activities like casual sex (at least before marriage) is that they are somehow displeasing to God.
But this is precisely where chaste fear steps in. We may feel that we desire some of these things, for example, that we desire to be able to have casual or even relatively committed sex before marriage. We may not understand how these things are inconsistent with loving our neighbors—but we still forgo these things, not because we fear the potential punishment or other consequences; rather, we forgo them because we recognize taht they are displeasing to God. Saint Augustine tells us that the one who refrains from sin because he fears punishment has not won victory over the sin:
“In vain, however, does any one think himself to have gained the victory over sin, if, through nothing but fear of punishment, he refrains from sin; because, although the outward action to which an evil desire prompts him is not performed, the evil desire itself within the man is an enemy unsubdued….
He, then, is an enemy to righteousness who refrains from sin only through fear of punishment; but he will become the friend of righteousness if through love of it he sin not, for then he will be really afraid to sin. For the man who only fears the flames of hell is afraid not of sinning, but of being burned; but the man who hates sin as much as he hates hell is afraid to sin. This is the “fear of the Lord,” which “is pure, enduring for ever.” For the fear of punishment has torment, and is not in love; and love, when it is perfect, casts it out” (Letter 145 to Anastasius).
An alternative analogy may be useful here. Saint Augustine refers to the difference between chaste and unchaste fear to be as the difference between a wife who fears to commit adultery because she wants only to not be displeasing to her husband, and a wife who refrains from adultery because she fears of the consequence of her husband’s anger. The one who has unchaste fear is afraid of being condemned, but the one who has chaste fear is afraid only of being forsaken.
This analogy can be furthered a bit. Suppose that a young man decides to court a young woman. He discovers that she has a phobia of spiders, and thus abstains from wearing his lucky “spider tie” on their date. He knows that she would probably forgive him for it—she might not even really be angry at him for it—but he does not want to displease her by his wearing it. Moreover, he is consciously alert for spiderwebs in the branches of the trees which the pass as he walks her home after dinner, even though he himself actually doesn’t mind spiders (and even though said branches are clear o the sidewalk on which they are walking). He does not fear that she will blame him should a spider alight upon her shoulder, but he does fear that she will be very unhappy nonetheless. This young man has a sort of chaste fear for the sake of the young woman he loves, because he “fears” displeasing her. But note that this “fear” is nt so much a matter of being afraid, but rather is a matter of loving her and taking utmost care to not displease her.
We can take “chaste” fear a step further. We ought fear not only that we will do something which is displeasing to God, but rather that we will fail to do something which is actually pleasing to Him and which was in our power to do. As St Gregory the Great writes in his Moralia, “To fear God is to omit none of the good that is to be done. Thus it is said through Solomon: ‘The man who fears God neglects nothing.’….The holy church of the elect enters on the path of innocence and righteousness in fear, but comes to its end with charity…every vice is crushed by fear, but the virtues spring from charity ”
Since we are imperfect in this life, we will fail at both, committing sins of commission and of omission. Here holy fear is for us, that we will be more alert to to possibility of doing wrong, so that we may avoid it (imagine if the young man above did not bother to find out that his love suffered arachnophobia!), but also that we will be more alert to the possibility of doing right so that we can please God with our good deeds.
Therefore, what does it mean to say that perfect love drives out fear? Saint Augustine gives us this answer:
“What then of the two fears? There is a servile fear, and there is a clean [chaste] fear: there is the fear of suffering punishment, there is another fear of losing righteousness. That fear of suffering punishment is slavish. What great thing is it to fear punishment? The vilest slave and the cruelest robber do so. It is no great thing to fear punishment, but great it is to love righteousness. Has he, then, who loves righteousness no fear? Certainly he has; not of incurring of punishment, but of losing righteousness. My brethren, assure yourselves of it, and draw your inference from that which you love. Some one of you is fond of money. Can I find any one, think you, who is not so? Yet from this very thing which he loves he may understand my meaning. He is afraid of loss: why is he so? Because he loves money. In the same measure that he loves money, is he afraid of losing it. So, then, some one is found to be a lover of righteousness, who at heart is much more afraid of its loss, who dreads more being stripped of his righteousness, than thou of your money. This is the fear that is clean— this [the fear] that endures for ever. It is not this that love makes away with, or casts out, but rather embraces it, and keeps it with it, and possesses it as a companion. For we come to the Lord that we may see Him face to face. And there it is this pure fear that preserves us; for such a fear as that does not disturb, but reassure. The adulterous woman fears the coming of her husband, and the chaste one fears her husband’s departure” (Tractate 43 on the Gospel of John).
In other words, love drives out servile, unchaste fear, but it leaves us with chaste, filial fear. We are no longer afraid of the punishments of God , but only fear that we cannot please Him enough, or that we will displease Him. Saint Thomas Aquinas adds to this that even servile fear is not entirely cast out by charity (ST II-II 19.4, 10), though its servility is; but that love does perfect servile (unchaste) fear and mold it into filial (chaste) fear. Indeed, filial fear will remain with us in heaven as a fear that “holds fast to a good which we cannot lose” (ST II-II.19.11). Love does not cast out fear, in other words, but instead perfects it and then is in turn further perfected by it.
 From his essay, The Rarity of the God-Fearing Man. In that same essay, he also notes that
There are things which rightfully we ought to fear, if we are to enjoy and dignity as men. When, in an age of smugness and softness, fear has been pushed temporarily into the dark corners of personality and society, then soon the gods of the copybook headings with fire and slaughter return. To fear to commit evil, and to hate what is abominable, is the mark of manliness. “They will never love where they ought to love,” Burke says, “who do not hate where they ought to hate.” It may be added that they will never dare when they ought to dare, who do not fear when they ought to fear….
“Forgetting that there exists such a state as salutary dread, modern man has become spiritually foolhardy. His bravado, I suspect, will stand the test no better than ancient Pistol’s. He who admits no fear of God is really a post-Christian man; for at the heart of Judaism and Christianity lies a holy dread. And a good many people, outwardly and perhaps inwardly religious . . . today deny the reality of reverential fear, and thus are post-Christian without confessing it.
Christianity always was a scandal; and I rather think I began to fear God because I discovered that terror to be so unconventional, impractical, and off-color in our era… Before I began to think much on the spiritual diseases of our century, I revolted against the disgusting smugness of modern America—particularly the complacency of professors and clergymen, the flabby clerisy of a sensate time.”
He does, however, acknowledge that there can be an excess of fear (by which he means servile fear), and that taken in the extreme this too can cause problems. As Monsignor Pope notes, “A mature fear of the Lord does not box us in or paralyze us. Rather, it reminds us of our boundaries and keeps us away from truly dangerous things that erode our talents. But because we love God, respecting His boundaries is a joyful thing for us.” This “mature fear” is what St Augustine means by “chaste fear” and what St. Thomas means by “filial fear.”
 Pope Benedict XVI went so far as to state that
Thanks to fear of the Lord there is no fear of the evil that rages in history and one takes up again with vigor the journey of life, as the prophet Isaiah declared: “Strengthen the hands that are feeble, make firm the knees that are weak, say to those whose hearts are frightened: Be strong, fear not!'” (Isaiah 35: 3-4).
The fear of the Lord therefore drives out worldly fear.
 Sermon 15 on the New Testament, which is specifically about the words of Christ recorded in Matthew 10:28: “And do not fear those who kill the body but cannot kill the soul; rather fear him who can destroy both soul and body in hell.”
 And perhaps the one found at the beginning of Matthew 7, which is arguably the most widely misconstrued passage today.
 Note that “less obvious on the face” does not mean that they are mere impositions which cannot be accessed by the use of reason or arrived at as a consequence of rightly ordered love. Rather, it means that the reasoning might be more complex, might have more steps, or might be less easy to follow without reflection.
 Note well that even servile fear is not necessarily bad. It is still fear of the Lord, but it is a less perfect fear. See Saint Thomas Aquinas’ Summa Theologica, II-II.19.4: “Servile fear is good, but servility is evil.”