We can come to know that there is a God but reason alone. We can come to know what God is or is not by using reason to study the universe and determine what the artist is by virtue of the artwork. We cannot come to know who God is unless God reveals Himself to us.
Just like I can know that you exist because you are posting here. I can reasonably determine some things about you by the same means. I cannot know who you are unless you tell me more. No amount of studying you from the outside would tell me who you are if you did not communicate it to me yourself. I could guess a lot. But I could not know you unless you revealed yourself to me.
We are contingent beings. We cannot by our intellect alone comprehend fully the one we are contingent upon. To say otherwise would be to limit God. Catholicism does not allow for God to be limited. Our intellect is inferior to God’s, a necessity of being contingent.
If we are to know more about God than can be deduced by reason alone, it must be through Revelation — God revealing Himself to man.
God revealed Himself to mankind in His one perfect Word, Jesus Christ. Jesus Christ is the fullness of God’s revelation of Himself to man.
We know of the Incarnation because it happened in time and space. It is a real event in history. God sent His only Son. He died for our sins and was raised from the dead for our salvation. Praised be Jesus Christ, both now and forever!
The Trinity is the truth of the One true God. It was revealed through Jesus Christ.
Neither could be known except through an act of God, revealing himself to man. Neither defy what is known about God through reason. Neither could be known by the intellect alone.
Just as I could not know what relationships you have in your life, and which are important to you, and what value you place on family, or work, or religion, unless you were to tell me.