Virtually every system of knowledge begins on an a priori basis, that is, with one or more unproven or self-evident principles. For instance, the entire system of mathematics begins with the concept of quantity. Given two piles of rocks, it is observed that there is a difference between the two piles - namely, that one pile is bigger, one smaller. To account for the concept of quantity, of more-ness and less-ness, man starts counting things and measuring things, and before you know it, he's calculating the distance to the moon.
As another example, scientific thought rests upon the unproven assumption that the stuff of our universe behaves in a predictable and reliable manner, that there are physical laws at work. Man sees things fall, comes to recognize and formulate the law of gravity, and before you know it, he's explaining how the moon orbits Earth. The assumption isn't as obvious here; it must be taken almost on faith that the universe behaves according to certain laws. There's no particular reason that this should be so; the universe could just as well behave in a haphazard and capricious manner. As Albert Einstein once remarked, "The most incomprehensible thing about the universe is that it is comprehensible".
Well, then, are the principles of science worthless because they are based upon an unprovable assumption? Of course not. Having begun with an act of faith in the reliability of physical laws, we eventually come to have tremendous evidence to bolster this faith. The accumulated wealth of knowledge serves us very well, and does a wonderful job of explaining how things work. The pieces fit together, it all makes sense, there is internal consistency, and it seems to conform remarkably to reality as we experience it. We may therefore with good reason assume that the original assumption was valid.
Now consider the various systems of theological thought, also based upon unprovable assumptions. Oh, there are proofs for the existence of God, but these rest upon the a priori assumption that there is such a thing as provable truth. The nihilist rejects this assumption, and so he sees no proof. Of course, the nihilist is likewise starting from an a priori position which defies proof. There seems to be no other way; you have to start with an assumption of some kind, and forget (for now) about proving it.
So i begin with some assumptions, then see where that takes me, what system of thought can be built upon the assumptions; that will be my proof. I might start with the atheistic assumption that there is no god, and build a system of thought upon that assumption. In such a case, i find myself forced into a purely materialistic and nihilisitic system, one that does not explain or even consider such spiritual concepts as justice, love, or good vs. evil.
If i try the theistic assumption, that God exists and cares for us, i can at least begin to build a system that explains something of the meaning of life, that also incorporates the idea of good and evil, and the moral dimension of human life. Because this results in a better system of thought, i think that the starting assumption must be more sound.
At this point i would like to know more details about this God, and more about how we ought to live. This calls for a creedal system, in which the details are fleshed out. Of the major creedal religions, Christianity stands out for a couple of reasons. First, because it confirms and completes another major creedal system, that of Judaism. Secondly, it makes the fantastic claim that God has been here in person. This claim must either be debunked or be recognized as the most complete revelation possible. More on that shortly.
Likewise, within Christianity, one particular system stands out, that of Catholicism. Firstly, it possesses the most credible historic claim to being the one church going back to the Apostles, founded by Jesus Himself. Secondly, because it has the most trustworthy epistemology, the most solid authority.
The epistemology of Bible-based Christianity is pretty solid, and fairly trustworthy in its adherence to the authority of the Bible, but there are some problems here. For one thing, the existence of several thousand denominations, all claiming to be Bible-based, is evidence that this approach is prone to errors. Like the Ethiopian eunuch (Acts 8:31), how can the believer know which interpretation of Scripture is the correct one, unless there is some reliable authority to guide him? Secondly, the Bible itself doesn't teach the doctrine of Sola Scriptura, that Scripture alone is all you need. Indeed, it points to the Church as the 'pillar and bulwark of truth' (1 Tim 3:15 RSV). And even if Scripture did proclaim Scripture as the only source of revelation, that would be circular reasoning, wouldn't it? No, the one fundamental doctrine that the Fundamentalists left out is that Christ built His Church upon the Apostles, and that He gave His authority to them, which remains a clear mark of His Church today.
Note that this authority does not preclude the obvious sinfulness of many priests, bishops and popes throughout history. Authority does not depend upon sanctity. (Indeed, the fact that the Church survived all her corrupt popes and bishops, and never lost the Gospel, can be seen as evidence of divine protection.)
So, i start with these assumptions: God exists, and reveals Himself to us in the person of Jesus who gives authority to men, thus establishing the Church. This produces a system of thought that answers an amazingly wide berth of life's questions, and answers them in an internally consistent way that makes sense of life, and corresponds extremely well to the realities around me. The Catholic system addresses not only specifically religious questions, but also issues of economics, politics and other things touching the demands of faith and morals. If anyone can come up with a more complete system of thought, i would truly love to hear about it. For now, this seems to be by far the best game in town, and explains why i am Catholic.
To return now to the startling assumption that forms the basis for all Christian faith - that Jesus is, in fact, God in the flesh - what to say about that? The veracity of this claim would seem to be tied to the Resurrection. If that man who was crucified really did rise victorious from the grave, then he must be taken seriously indeed, and not merely as a good teacher. Power over death would certainly seem to indicate a superhuman nature. If Jesus didn't really rise, then he was just a man, the whole thing is fool's work, and i would have to reject the Christian way, probably reverting to a creedless theism. I would be left to explain why the early disciples were all so willing to suffer death by martyrdom for a hoax, and i would still have to admit that the Christian system of thought blows the theistic system out of the water. But it needs to be said that, without the Resurrection, the Christian Faith makes no sense at all.
It comes down, then, to this: I have very strong intellectual reasons for embracing the Catholic Faith; other systems of thought simply pale by comparison. But in the final analysis, it still has to be a step of faith. I believe Jesus really rose from the dead, and so i am Catholic.