The first “Christian” were were originally monotheists. They were the true apostles of Jesus whom some call the Nazarenes.
The modern day Christians originated with Paul in Antioch. That’s where and when the followers of Paul started being called Christians.
From Acts 11:26 we read that was about 30 years after Jesus’ crucifixion. All this time the Nazarenes were headquartered in Jerusalem and coexisting peacefully with mainstream Judaism. When Paul showed up in Jerusalem preaching that Jesus was God, and that he had resurrected, he was nearly killed by the regular Jews for preaching heresy because the Nazarenes had no idea of those things about Jesus preached by Paul.