People who believe the Trinity is a mystery have never read their bible cover to cover.
Christianity is NOT a mystery religion.
Read Romans 1:20 - what it says about the Godhead, and what you can know about it, and who can know it.
You either believe Romans 1:20, or you believe the Trinity is some mystery. There is no reconciling that verse with the belief we cannot grasp the nature of God. The problem is not so much that God is hard to find, the problem is that Mankind loves the dark, and it’s own ways instead of the things of God.
What are the things of God? Read Romans 1:20.
By the way, both Catholics and Protestants need to read their bible, and believe it instead of commentaries and pronouncements. Why take 2nd hand stories when you can go to the source?
Romans 1:20 is talking specifically about natural knowledge of “God’s eternal power and divine nature,” not the knowledge of the Trinity that was only revealed by Jesus when he taught his disciples “that I am in the Father, and that the Father is in me” (John 14:10). If you “read your Bible cover to cover”, there are plenty of references to divine mystery, some using the actual Greek term “mystery”—e.g. Ephesians 3:3 (”the mystery made known to me by revelation”), 1 Corinthians 14:2 (”he utters mysteries with his spirit”), 1 Corinthians 15:51 (”Listen, I tell you a mystery”), etc.—others using other vocabulary to convey the concept that “no one knows the thoughts of God except the Spirit of God” (1 Corinthians 2:11). This does not make Christianity a “mystery religion” in the sense of the pagan mystery religion. It simply means that God’s eternal knowledge includes more than human reason can attain without revelation, as exemplified when Jesus told Peter, “Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by my Father in heaven. And I tell you that you are Peter, and on this rock I will build my church. . .” (Matthew 16:17-18)