The Trinity is not in The Bible, neither is the Nicean Creed yet “Christians” ignore that fact.
The DOCTRINE of the Trinity is in the Bible, even if the word isn’t. Interesting to note that the LDS use the word ‘atonement’ which wasn’t invented until the 16th century yet get caught up on the word Trinity.
The Trinity is certainly mentioned in the bible...not bu that name, but reference to the Father, Son and Holy Spirit are certainly there and to simplify referring to the three persons, Trinity came into common useage. The Nicene Creed is merely a compilation of what Christians believe....all the beliefs are in the Bible, the creed allows us to state our beliefs in an organized manner. Read it, you will learn a lot!!!