The most profound mystery of our Faith is, of course, the Trinity. The Trinity tells us-as much as we can understand-who God is. And even though it is very hard for limited, finite, fallible human beings to understand that which is infinite, infallible and unlimited, God, we can still know something about Him.
First of all, simply by using your reason you can know that He exists. Just thinking about God on the beach one day you can conclude that there must be some Being who began everything.
And then you make the second conclusion that the One who began everything, the One who created everything, is Himself uncreated. You realize that no one could have created the Creator. Because-if the Creator had been created-then who created that one? And that question makes no sense.