How did we come to believe that there are three persons in one god?

L Christians believe the doctrine of the Trinity. If you do not believe this—that is, if you have come to a settled conclusion that the doctrine of the Trinity is not true—you are not a Christian at all. You are in fact a heretic. Those words may sound harsh, but they represent the judgment of the Christian church across the centuries. What is the Trinity? Christians in every land unite in proclaiming that our God eternally exists as Father, Son, and Holy Spirit. Those who deny that truth place themselves outside the pale of Christian orthodoxy.
Having said that, I admit that no one fully understands it. It is a mystery and a paradox. Yet I believe it is true.
I can think of at least three reasons for believing in the Trinity:
The Bible teaches this doctrine.
Christians everywhere have always believed it.
No other explanation makes sense.
Someone has said it this way: If you try to explain the Trinity, you will lose your mind. But if you deny it, you will lose your soul. Let's take a look at the defintion of the Trinity and what the role of the Trinity is today