How do you get from this:
Of course it's ultimately a mystery; created intellect cannot perfectly grasp or fully understand the relationships within the Godhead.
... to what you said? "Accepting developed doctrine on its own merits" won't get you to a perfect understanding of the relationships with the Godhead either, because your mind simply isn't capable of it. No human being's mind is.
Do you think you have a perfect and complete understanding of nature? If so, you're wrong. Do you think any human, even the most brilliant scientists on earth, have a perfect and complete understanding of nature? Wrong again. (And those scientists would be the first to admit it.)
Guess what ... God created nature. He is therefore bigger and harder to understand (intellectually) than nature, because the lesser cannot create the greater.
QED. How you get from the inability of created intellect to perfectly comprehend the infinite Divine to "just accept developed doctrine on its own merits" I'll let you try to explain.
The Catholic Church developed and formalized that idea along with the Nicene Creed whic is also not cited or to be found in the Bible.
As well as every other doctrinal statement about orthodox Christology. I have no idea why anyone would be surprised that a creed written to summarize the Bible (on one short list of topics only) wouldn't itself be in the Bible. That's like complaining that the Cliffs Notes for "Hamlet" weren't written by Shakespeare.