Can the mystery of the Holy Trinity be known by human reason alone?

Answer 1

Yes. The concept of the Holy Trinity was developed by humans, ordinary in intellect in spite of their vocation, during the late third century and the fourth century CE. The outcome was the result of spirited debate, discussion, negotiation and argument over a period of decades.

As to whether the concept was arrived at by divine inspiration, in fact it was proposed by Bishop Alexander at the Council of Nicaea, against strong opposition, and was almost abandoned over the ensuing decades. Clearly the Church leaders were annunciating beliefs, not inspiration. If God had wanted to guide the Church leaders in understanding his true nature, he would surely have guided them in the same path. It was Emperor Theodosius, a layman and secular leader, who at the end of the fourth century finally decreed that the matter was settled, and that Jesus was of one substance with God. The filioque clause, a relatively minor detail concerning the Holy Ghost, was never agreed on by the Church as a whole, demonstrating that this too was a human decision, not a divine one.

There is no mention of the Trinity in The Bible, with the closest apparent reference being the "Johannine Comma" (1 John 5:7), which refers indirectly to the concept of the Trinity. This verse was not in the earliest Greek manuscripts until it appeared in a Latin copy in the fifth century.

So if the mystery of the Most Holy Trinity can be devised and understood by Christians in the fourth century, then it can be understood by Christians today.

ANSWER

for my own understanding, God has left some traces of his trinitarian being in creation and in the Old Testament but his inmost being as the Holy Trinity is a mystery which is inaccessible to reason alone or even to Israel's faith before the Incarnation of the Son of God and the sending of the Holy Spirit. This mystery was revealed by Jesus Christ and it is the source of all the other mysteries.