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)
God’s Triune nature is, if you really believe Romans 1:20, defined in General Revelation (that’s what Romans 1:20 is talking about).
Yes, there are mysteries (we know not the time or hour) but they are NOT mysteries of things we have need to know of. Christianity is not a mystery religion.....we do not believe in a mystery. However there may be some mysteries about our beliefs. So you are right quoting about the mysteries, but that is not a mystery of the nature of God.
Why does man have such a time understanding the Trinity?
Let’s look a moment.
God exists as three in one. That is, he is both Three and One. And the oneness is indivisible from the three-ness. Jesus existed, The Father existed, and the Holy Spirit existed forever, and will forever. They had no beginning or end. And yet they are one.
Now let’s look at our physical universe (why, because that’s what Romans 1:20 says speaks to it).
The universe has 3 physical dimensions. Each is infinite, and without all three the volume of the universe would be zero, it would not exist.
In addition time, which is not a dimension of the universe btw, has a past,present and future.
Everything in the universe is essentially three in one at it’s core nature.
Now, some accuse Christianity of worshipping “3” gods. This can no more be the case than you could divide the universe into 3 “parts”.