In a recent post we saw that we need revelation. In SCG I:4 St Thomas goes into a bit more detail as to why we need it if we are going to know God. He assumes here of course that this is something that we must do and that we want to do it. He has addressed these points in a brief fashion in prior chapters, and he’ll give a fuller treatment of them later on; in short, we must do it because we were made for this purpose, and we want to do it because God is perfect, and the contemplation of perfect Truth is what knowing God consists in. Who wouldn’t want to know God once he knows that He is his Maker? But as I said—more about these matters will be forthcoming later in the book.
At this stage, although he intends to present an argument from reason for the truth of the Catholic Faith, he wants to make sure that certain other points are understood from the get-go: the Christian Faith is not a creature of reason, and God has not left it to us to figure Him out. Rather, He has revealed Himself to us, and He has done so because of our weakness.
 Yet, if this truth were left solely as a matter of inquiry for the human reason, three awkward consequences would follow. [I:4, 2]
The first of these consequences, he says, is that few men would ever know God [ibid., 3]. Why? Because most men aren’t able to spend the time that is necessary in the study and thinking that is necessary for them to know God at all solely by means of reason. They lack the physical gifts for it, or they have so much to do just to make ends meet or to feed their families that they can’t do it, or they simply aren’t interested in doing so, says Aquinas: you have to know a lot of other stuff before you can get to the knowledge of God by means of reason. There is a lot of philosophical preparation that’s necessary, and most people won’t make that effort. So very few people would come to know God at all if it were a question of human effort.
The second of the consequences, he writes, is that it would take too long to get that knowledge. It takes years of study. Not only that, but young people are not generally disposed to this sort of pursuit: there are many other things that interest or intrigue them, so that they don’t bother so much with the mental effort that’s necessary to learn about God by means of reason. The upshot is that knowledge of God would be limited to those few more mature people who have the resources and the interest in seeking Him that way.
The third awkward consequence is that human reason is prone to error. We’re not perfect, and for all sorts of reasons we make mistakes. This means that if our knowledge of God was solely available to us by means of reason, the most likely scenario is that we’d wind up believing many erroneous things about Him.
For all these reasons, then, it’s not sufficient for us to rely solely upon what we can figure out on our own. We need God to reveal Himself to us, and He has done so in His mercy. In this way any one may know God by the assent of faith, because faith does not depend upon reason. Still, this does not mean that the Christian Faith is contrary to reason at all; it simply means that God in His mercy has condescended to help us to find Him when we would otherwise not be able to find Him at all. And of course it is also true that there are many truths about Him that we could not discover by means of reason even if we were as smart as St Thomas and had all the time in the world for study and never made mistakes. We can’t save ourselves.
A related point is that if we can’t reliably, easily, and readily know God by means of reason, then other human means are even less helpful. Some folks say things such as “I like to think of God as…” But what we like, and what seems pleasant to us, is dreadfully unreliable as a measuring rod for the truth. We can’t trust our feelings as a guide either for knowing truth generally or for knowing truth about God. Pace the Mormons a burning in the bosom is nothing but subjective and tells us nothing about what the truth actually is.