PSac, I agree in part and disagree in part. The teaching is not explicit in the NT, though the habits of the primitive Church were clearly Trinitarian in nature. The term obviously post-dates the NT, but that is not the same as saying the doctrine was imposed as a late addition.
PSac: it is NOT required for believing Christ to be our Lord and savour, Son of God, and that he was resurrected.
People can believe lots of amazing things without any particular reason. I do think that the early Councils were correct to observe that the core Christian claims about Jesus being the Son of God, that he was raised, that his death was in order to fix creation, must necessarily imply the Trinity. Logically, then, if P implies Q, then denying Q means denying P.
So, I think it really is necessary. Which is not to say that someone could not
accept the Christian premises and still reject the Trinity. But that's a different sort of question, I think. They couldn't do it logically.