Anybody that is familiar with Rene Descartes is familiar with his "clear & distinct ideas". In attempting to construct a perfect science, Descartes tore down the structures of philosophy and theology that preceded him, and sought to build a perfect science based upon clear and distinct ideas. So, he found that the clearest and most distinct idea that he could discover, that which he could not doubt in any way, was Cogito ergo Sum. I think, therefore, I am.
Jacques Maritain, in the book Three Reformers, quotes Bossuet as follows: "Under the pretext that we must not accept anything but what we understand clearly - which, within certain limits, is very true - everyone gives himself liberty to say, 'I understand this, and I do not understand that,' and on this sole basis they admit and reject whatever they like. (p. 75)"
There are many things that we, as humans, and due to our natural limitations, cannot understand. Some such things include what it means to be eternal, how it is possible for Jesus to be at the same time fully God and fully man, what it means to say that God is triune. There are, perhaps, many more subjects that we cannot fully understand. However, there is a difference between something being a subject that we can't, due to the limits of human nature, understand, and something being a subject that is very difficult to understand. There are an innumerable number of subjects that are so difficult to understand that only a few humans may ever attain an understanding of them, but, these subjects, though not easily understood, are not, for all that, beyond our reach.
A disturbing trend that I have noticed in the church is summed up by the following phrase, "X is beyond me. But, there are some things that we were not meant to fully comprehend..." (X is a place holder into which you can insert any number of doctrinal or philosophical subjects.) The remark by Bossuet is directly to the point, just because I don't understand something doesn't mean that I can reject it as false, or put it in the pile of the impossible, or mysterious. (This seems to be one of the many ways in which Descartes has influenced the contemporary church.) On the contrary, we should not allow our personal incomprehension of any given subject to get in the way of our potentially learning some vital truths. The joy of being human, of being rational, is the joy of discovery.
Those things that the human mind is unable to discover, which are necessary for our salvation, have been revealed to us, by God, in the Bible. Aside from these things, which we cannot arrive at by deduction, we should not count anything as being incomprehensible. If something is difficult to understand, let us roll up our sleeves and seek truth.