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”.
With respect to Romans 1:20, I would suggest that the problem in the historic discussion of this verse stems from the usage of the English term "Godhead" in some translations to render the Greek word "theiotes", which also appears as "divine nature" in other translations. This word was essentially an older English pronunciation of the word "godhood" and, as such, carried the root sense of "god-ness" or "divinity." However, since at that time English speakers also generally assumed that God was Trinitarian in nature, the same English word could also refer to God's Trinitarian nature. In Webster's listing for "Godhead" these definitions are distinguished as two distinct meanings. In the ancient Greek used at the times Romans was written, however, the word "theiotes" simply carried the first meaning of "Godhead" in the general sense of "divine nature", using a root word that went back to Homer and had been adopted by later Greek and Roman writers. The word Paul uses is also used by Plutarch, for instance. On this I would reference Liddell and Scott's Intermediate Greek-English Lexicon.
With respect to Romans 1:20 citing the physical universe as a reflection of God's nature, I would observe that Paul was writing before Descartes introduced the modern coordinate system analyzing the universe in terms of three dimensions, and Paul's contemporaries would not have understood his words as referring to a three-dimensional model of the universe. Paul specifically referenced two aspects of God's nature reflected in the universe: 1. His eternal power 2. His divine nature. The relevant question is what he was alluding to by these terms and, specifically, whether he was talking about the Trinity when he used "theiotes". I would suggest that in context he was alluding to Greek philosophical discussions of God's existence, which were familiar to Hellenstic Jews of Paul's time such as Philo of Alexandria, and were echoed by other early Christian writers like Justin Martyr who referenced Greek philosophy and literature when engaging in apologetics--as Paul does in Acts 17:28 when he alludes to the poet Aratus' Phaenomena.