For those who believe the Bible is the final authority for Christianity, there are many doctrines that are not found in the Bible. One of them is the Trinity
The Bible
Yes, the Bible does talk about the Father, Son and Holy Spirit. But in no place does it say that these three persons are of one essence.
Four Sources of Authority
But for Progressive Christians, like myself, this is not an issue. The Bible is not the final authority for Progressive Christianity. Rather we have four sources of authority. They are Scripture, Tradition, Reason, and Personal Experience.
We get the doctrine of the Trinity from Tradition, not from the Bible. Yes, the doctrine harmonizes with the Bible, but its source is from Tradition.
Tradition
I capitalize the word Tradition because here I’m speaking specifically about church Tradition that is virtually universally accepted. That is, there’s a general consensus about the doctrine over a long period of time.
But Tradition is not the final authority. The final authority is the church, the Christian community itself. It is the church that defines and interprets those doctrines that are authoritative. And it negotiates these based upon the four sources of authority, which again are Scripture, Tradition, Reason, and Personal Experience.
Reason and Culture
And just to quickly clarify, by Reason I’m including science, history, and modern scholarship, as well as logical and reasonable arguments.
The general culture of a society can inform the church to renegotiate issues that may have been misinterpreted. At no time is the doctrines of the church infallible or inerrant. They are always open for negotiation based upon new evidence and new information.