Actually, the first known use of the Trinity referring to the Godhead- was by Theophilus, Bishop of Antioch-EdselB wrote:The term Trinty was not an invention of Nicea. Tertullian used it as well as the terms "substance" and "economy."
I have found helpful the lecture by D. Bercot. "What The Early Christians Believed About the Trinity."
It is available at:
https://www.scrollpublishing.com/store/product133.html
This is who he was:
https://orthodoxwiki.org/Theophilus_of_Antioch
And this is WHY he used the term- he was dealing with Autolycus an Idolater and Scorner of Christians:
http://www.piney.com/HsTheopTrinity.html
I completely disagree with the implication that this is 'adding to Scripture', but as JM conveyed- it is necessary to help convey the understanding to those who attacked Christianity with heresy-
I agree this is an Anabaptist forum, but- because the Anabaptists were not there when all of this took place and focused on their own doctrines in writing up their particular 'confessions of the faith'- that if a question comes up to try to understand the Trinity, why this doctrine was articulated, it is better to go back to those who actually were the Church where the origin took place- it's difficult to start in the 15th century and look backwards from all the Protestant groups.
It definitely is not adding to Scripture, it's teaching is using Scripture PLUS the understanding the Apostles had through Jesus own words, and revelations of the Holy Spirit-and it was used to deal with the heresies that were trying to destroy the Church.