Just a curiosity - where in the bible does it say that onbly biblical terms can be used about matters in the bible?
It doesn’t. This sounds like some splinter Protestant group.
If you look at the early Church Fathers - those men who were trained directly by the Apostles, or trained by the first group who were trained by the Apostles, you will find discussions about the trinity.
Why is that? Because there was no bible at that time. The books in the bible were not agreed to until well into the 300’s. The writings of Paul, James, John, Peter, Matthew, Mark, Luke and John were circulated; but there was no bible per se. Early on there was agreement in large part of what we would eventually find in the bible, but the book itself didn’t exist except in its separate parts.
As questions came up which were not specifically answered in those writings, or what was eventually agreed upon as the bible, the Church answered those questions; and so later as there was a split in authority over what the Trinity meant, those questions were answered by the Church, which was given that authority by Christ.
And the root of many questions has been the failure of some people to accept that authority of the Church to define matters.
We have sola scriptura - bible only - which is a rejection of the Church to answer questions authoritively. The result of that is 30,000 divisions within Protestantism, give or take a few thousand. And nowhere in the bible does it say that the bible is the sole authority. In fact, it gives authority to Peter, which was acknowledged by the early Church (and reported in the bible) and the early Church, after Peter died, continued that authority on in the bishop of Rome - whom we call the Pope - or in the vernacular - Papa.