I've just recently come to the conclusion that Christianity began as a mystery religion. I was somewhat familiar with the NT when I first learned about the mystery religions and as I started to learn about them, it occurred to me that the NT had many passages that seemed to echo what the mystery religions were teaching. Just as an example, I read David Fideler's book, Jesus Christ: sun of God... and he mentioned how the Outer or Lesser Mysteries were called "milk for babes" and the Inner or Greater Mysteries were called "meat for men" (I'm not quoting the book), and then I found in the NT in 1Corinthians Ch. 3,
"Brothers and sisters, I could not address you as people who live by the Spirit but as people who are still worldly —mere infants in Christ. 2 I gave you milk, not solid food, for you were not yet ready for it. Indeed, you are still not ready.
There are more examples in this video on the subject.