I used the following when teaching on the Trinity a few years ago.
How old is the Trinity?
How does one explain this central mystery of Christian faith which transcends human reason, but is not contrary to it? Especially to someone who is not Christian.
Dogmas of the Christian Church are contained in the Original Revelation. For the Church and its magisterium recognize that they are not the mediators of Revelation… but only guard, transmit and explain the Divine Revelation which came in Jesus Christ at a given point in past history.
For Revelation itself has a history, and necessarily so, not Just because the speaker, God, can in His freedom act historically, but because the hearer, man, is a historical being.
As long then as his history goes on, there must be a history of dogma, although revelation is closed. Then there are mysteries The many mysteries are comprised of truths which are provisionally incomprehensible. It is the Truths that are Mysterious.
The supreme Mystery – the Trinity – is also the most obscure. There is a true mysticism of the Trinity.
We ourselves have nothing to do with the mystery of the Trinity, beyond receiving some revelation “about it.” The references to the Trinity are many in both the New and Old Testaments. In Matt 28:19 Christ commissioned the apostles to “Baptize in the name (singular) of the Father and of the Son and of the Holy Spirit.” The Athanasian creed does a very good job of setting forth the Trinity in our limited human language. In John 10:38 Jesus says " I am in the Father and the Father Is in Me" in John 10:30 Jesus also says “the Father and I are One” note he does not say that He and the Father are the same or that He is the Father. John 14:7-11
In the places where the Father speaks from the clouds, the Father and Jesus and the Spirit are distinct from one another.
( MT 1 7:5, MT 3:1 6-1 7, Mk 1 :1 0-11 and lk 9:35)
In the Old Testament there are foreshadowings of the Trinity. The Mystery is revealed in the New Testament, those who do not accept the New Testament will have the same difficulties as those in
John 10:19 and 26.
The use of the plural noun “Elohim” and the plural personal Noun “Ahad” in Gen. 1:1, 2:4,
Gen. 1:26, 3:22 and 11:7 etc. The triple repetitions of the Divine Name as in Isaias 6:3.
In translation Gen. 1:26 says “let US make man to OUR image” and Genesis 3:22 “and He said: behold, Adam is become as one of US, …” In the Gospel of St. John there are several passages that imply the Father, Son and Holy Spirit are distinct persons. (Jn 1:29-35, 14; 16, 14:26 and 16:15)