You bring up a lot of interesting considerations, with many of which I agree. In the following I may repeat something of what I have already said, though in some variation. In my personal experience, the determination relates to both a man's interest in the truth for the sake of truth. - This is indeed on the part of man. And the sound reason has to take a certain part in it, lest a man be just confused by the various ideas of those, who have erroneous views, and yet claim that they have to be believed, though they cannot be understood.
On the other hand, it relates also to man's approaching the Lord, who is the Light. On still another hand, it relates to the goals of the Divine Providence, for the Lord, according to what I learn, does not necessarily allow men in this life to come to the genuine true (but this is another issue).
To summarize:
1. Looking to the Lord J.C., who is Light. (in some cases, simply looking to God, when a man does not even know about the Lord).
2. Affection of truth for the sake of truth (thus, not for the sake of status, loyalty, self-advantage in the world, gains), and thus also for the sake of God, and eternal life. That kind of affection/interest is certainly not given with many, but still there can be some approximation to it.
3. Sound reason. Hopefully it is actually sound, at least, to such a degree that a person does not subject on the other hand his thought to the merely sensual confirmations in the first place, but on the other does not allow the various methods of persuasion/authority to persuade him to see the white dove as the grey or as the black, by the skilled arguments. This results in observation whether someone's arguments are consistent, sound, and say the intepretation/method explains the various things in the Word without contradictions or not.
The above may sound challenging, but in practice it can be even more challenging.
Consider the case.
1. In the letter of the OT there is an idea of the three Divine Persons. At least, it appears that there are three persons. However, it is claimed that God is one.
2. In the letter of the NT there is an idea of the three, though it appears in some places that Father is separate from the Son, but in other places, it is stressed that they are one. In some places, it appears as if the Father is higher Divinity, as it were. But in other places, that they are essentially equal. In still other places, the focus shifts to the Divinity of the Lord, that He is the Door, that the One who sees Him sees the Father, that He was given all power in heaven and on earth, and that His Human was glorified, so essentially Divine. Besides, He was called by John in the epistle, the True God and Life Eternal.
So, there was one intepretation of Trinity as per Apostolic Creed. Nothing stated about three distinct persons-god.
There was the Nicean Creed, when the terms were introduced, though it was not specified in the very text, perhaps, with 100% clarity, how exactly the persona/hypostatis to be understood, whether as Person in the nowadays sense of the word, or Divine Attributes. And yet the idea was of course that there one Divine Substance.
Then the Athanasian Creed perhaps made those persons a bit more distinct.
Now, when we regard those ideas in the light of the Word, we cannot deny the Trinity, for the Father, Son and Holy Spirit are spoken about, and it would be incorrect to deny that. But how do we understand it? If according to the letter alone, then there appears to be three, or even distinct persons if per OT. Though the Holy Spirit stands a bit separate, for the Lord, having breathed on the disciples, passed the Holy Spirit to them, which is evidently not a Person (in the nowadays sense of the term).
So, if we just rely on the idea that they, while being distinct persons, yet constitute one God, does it agree with the Word in all places, and with sound reason?
But if we consider the idea, which involves not only the letter, but the spiritual sense, and is not opposite to sound reason, that the Trinity is not outside the Lord, but in Him, as the Soul, Body and Operation are constituents or attributes of One Person, then that interpretation may be more easily seen to be more sound on one hand, though it may not be clear, why there is such appearance of the Father distinct from the Son, in some places, as Individual Persons, and how to explain those.
So, how would a man, not knowing which interpretation is correct, act? Perhaps, study arguments of both sides, apart from prejudices, propagandas, appeal to authority and tradition?
"The Divine Trinity in one Person is to be understood as soul, body, and proceeding operation, which together constitute one essence, for the one is from the other, and therefore the one belongs to the other. In the same way there is a trinity in each man, which taken together constitutes one person, to wit, the soul, the body, and the operation that goes forth. But in man this trinity is finite, because man is only an organ of life; whereas in the Lord the Trinity is infinite and thus Divine, because the Lord is life itself even in respect to the Human, as He Himself teaches in John 5:26; 14:6; and also elsewhere." ("Nine questions")
