“The things which we Christians embrace by divine faith, and which are not to be received except by divine faith, are humanly incredible. They are incredible for two reasons, two reasons paradoxically opposed: first, because they are so far above us, because they make it plain that God is remote, infinite, and mysterious beyond all imagining; second, because they bring that inaccessible Being so close, involve the two of us in each other, show us how much we mean to Him who is above all, how each of us is the preoccupation of Wisdom Itself, as much as if God had no other one to car for” .
These words were penned by Dr. Charles De Koninck in an article about the importance of mystery to the Faith and about the dangers of taking the mysteries of our faith for granted. Familiarity may at times breed contempt, but a possible danger facing those of us who were raised in Christian home or general and Catholic homes in particular is that of complacency.
There are any number of mysteries to which we must give assent, but for those who have heard these mysteries repeated and explained as a matter of course since youth can easily forget that these are first and foremost matters of faith and not merely of understanding.
This is not to say that faith precludes all understanding, or that any amount of understanding precludes faith. Rather, it it is to acknowledge that there are some contents to our faith which are beyond understanding—these are mysteries, and they do preclude complete understanding.
A mystery is that which we cannot fully understand, or fully grasp, and yet there are some mysteries which we believe to be revealed to us by divine revelation, and as such they demand the assent of the faithful. They may seem at a glance to be contradictory, but closer examination and greater understanding reveals that are not self-contradicting.
The philosopher Edward Feser explains this respect to the mystery of the Trinity:
1. The Father is God.
2. The Son is God.
3. The Holy Spirit is God.
4. The Father is not the Son.
5. The Father is not the Holy Spirit.
6. The Son is not the Holy Spirit.
7. There is exactly one God.
…The Trinitarian theologian maintains that the Trinitarian propositions (1) – (7) listed above are perfectly consistent when rightly understood, so that if any reading of them seems self-contradictory, then that reading is mistaken, and does not accurately convey what the doctrine says. Hence if the doctrine “appears contradictory” to you, you have by that very fact misunderstood it and are not really entertaining it at all. At the same time, if you give these propositions an alternate reading on which their consistency is entirely transparent, you have no doubt fallen into some heresy or other.
What Dr. Feser is getting at is that the Trinity is ultimately mysterious, that is, beyond our total comprehension, yet at the same time it is not a doctrine which is completely unbelievable. The doctrine cannot be proved, can not even be shown and thus known in the same sense as, say, the existence of God per St. Thomas’ Five Ways . As the bishops note in one of their catechisms , “The mystery of God is not a puzzle to be solved. It is a truth to be reverenced. It is a reality too rich to be fully grasped by our minds, so that while it continues to unfold, it always remains mostly beyond our comprehension.”
That is to say, the Trinity is a mystery which belongs to the deposit of our faith, and this doctrine (dogma, in fact) must be believed by all Catholics, yet we cannot understand it fully. Nevertheless, we can contemplate it, and we can come to understand the doctrine more fully —for example, by learning what it does not entail, as is so often the case .
This very mysterious doctrine may be called the center of our religion as Catholics, indeed as Christians. Nor can we deny that other central mystery, which is the the Incarnation. These mysteries may be rightly called, as one of them is called by St. Paul, a “stumbling block”, a “folly,” or even a “scandal” (1 Corinthians 1:23).
Herein lies the very dilemma of the Faith. We are bound to believe all that is revealed to us, and must accept it on faith and on authority. We lose sight of the difficulty in accepting the truly mysterious, precisely because it is familiar. On the other hand, too many abandon these mysteries, precisely because they cannot be fully grasped (to say nothing of proved).
We all want a sense of certitude where none is granted, and we all want to place a limit on what we must accept without proof. As Dr. De Koninck notes elsewhere in his essay:
“Our Faith is assuredly no easy matter and can move us to protest. Not only because it tells of mysteries which eye hath not seen, nor ear heard, nor mind conceived; but also because it penetrates every corner of life, because it will leave no part of our day untouched.
We all hold the instinctive attitude that there ought to be some blessed limit to what we are asked to take on faith, some acceptable frontier.
But where are such bounds to be set? Shall we confine our assent to the Incarnation, for example, with no reference to the Child’s mother or foster father? If we could manage these matters to suit ourselves, we would all feel inclined to suppress facts such as that God was hungry, tired, thirsty, that He perspired, that He rode on a donkey, that He died.
Yes, let divine truths be as lofty as you please; let it be glorious, sublime, awful, but let it not become human, pedestrian, ordinary, just like me and my dull little life, for then it shocks the intellect.”
Of course, that knife of limitations cuts both ways. There are plenty who will believe that Jesus lived and died as a Man—that is, that He did all of these things which make up the equivalent of each of our own “dull little life”—and who conclude for that very reason that He was nothing more than a Man. They note that the Incarnation of God as a Man is impossible simply because that Man did so many things that ordinary men do.
We might well be shocked that He did—yet what more evidence could we ask for concerning His humanity? You can’t win them all.
 Charles De Koninck, “Our Awesome Creed: The Faith is no excuse for bigotry,” Saint Joseph Magazine, Vol. 65, no. 10, pp. 16-19 (1964).
 Summa Theologica I.Q2.A2-3. Note that although St. Thomas’ demonstrations are often translated as proofs, they are only meant to show as far as human reasoning can go that God exists. There is a limit to human reasoning, and the Five Ways are often misunderstood above and beyond this. The proofs are discussed in more detail on, for example, Dr. Feser’s blog, and Mike Flynn is currently in the process of
 US Catholic Catechism for Adults, pp. 51.
 An analogy can be made here between our faith in the eternal mysteries revealed to us, and (of all unexpected thing) of science. Both have their beginning points—in the Faith, it is a revealed truth which is the starting point, in science a few assumptions such as the reliability of observation and method; both then have a nearly limitless exploration which can be undertaken, and neither can ultimately lead to complete understanding of the thing contemplated. We have our mysteries, and we have Godel’s theorem.
 For example, we can learn what the Church teaches against such heresies as modalist-Sabellianism, Arianism, Nestorianism, etc. These various heresies make one or another oversimplification concerning one or more of the Persons of the Trinity, or about the Trinity as a whole. See also Summa Theologica I.Q2.A2-O2.