The mysteries of the Kingdom of heaven… (Matthew 13:11)
Here’s a mystery for you: We come equipped with our five senses. By them, we attain knowledge. Mosquitoes, water, leaves on the trees, chicken-salad sandwiches, other people, etc… We know about all these because we have five senses by which to perceive their existence.
It’s impossible to imagine knowing anything other than what we perceive through our five senses, and then analyze. Granted, things like 2 + 2 = 4 are abstract knowledge, which does not require seeing or hearing or tasting or feeling or touching anything. But it is impossible to imagine that we could know that 2 + 2 = 4 without having first seen two apples, and then two more apples, and counted four apples. Or two Legos, or two baseball cards, or what have you.
So, the mystery: We have knowledge because we have five senses. But the one thing we exist in order to know cannot be perceived by the five senses, under any circumstances that we know of.
God brought us into being so as to know Him and, by knowing Him, love Him. But God we cannot see, smell, hear, touch, or taste. Everything that we see, smell, hear, touch, or taste is less than God. Because everything we can perceive is something that God made, like He made us.
(Remember, the most-important idea ever: there are two basic categories. The Creator and the created.)
Feel me? Maybe not. This is called via negativa—acknowledging to ourselves that the one thing truly worth knowing is the very thing we absolutely, positively do not know.
That said, the via negativa is not the only via. There’s another via, by which we can, in fact, know God somewhat, during our pilgrim lives.
The unknowable God became man and dwelt among us. The tidings of His life have reached us, by word of mouth.
So, while sight, smell, taste, and touch still have to follow via negativa very strictly; while we still have to say to ourselves that our chicken-salad sandwiches, no matter how delicious they may be, are not God; while we still have to exercise great discipline in this area, we do, in fact, have one sense that can give us solid knowledge of God: hearing.
When we believe what we have heard from the Apostles, then we truly know God. (And, just to clarify, reading counts as hearing, not seeing. Reading is like a secondary way of hearing.)
To put it in a nutshell: what we read in Scripture and the Catechism gives us knowledge of the one thing truly worth knowing, God Himself.