I remember hearing a while back that it started as a branch of Judaism (that might also not be true) and that many at the time (the Romans I think) saw it as a cult of sorts and it wasn’t widely accepted for many years. Now everything I’ve just said may be completely wrong.
(Also please don’t go in the comments and start mini wars of what religion is better. Nobody’s got time for that)
I think you’d have to define your terms to get a good answer to this question. What is the difference between a religion and a cult?
Christianity certainly did start as an offshoot of Judaism and grew over time until it was much larger. Christians believed that the Jewish messiah had come in the form of Jesus Christ, while mainstream Jews were still waiting.