Well, I have to say that didn’t surprise me. The Bible, but more specifically is written after the death of Christ, merely 60years after, and report or the reports about what Christ said to his disciples… That pre-supposes disciples had excellent memory or they all know to read and write… At this period, big doubts… Then you have the problem of the report of what somebody else said.
So the fact that Christianity picked in other Religions (Mazdaism, Mithra etc) isn’t surprising. After all, the Christianity is flexible enough to include Xmas, St John and different others Pagan celebration, and to build Churches where miracles due to others Gods did happen…
The fact that you don’t have the direct writings of Christ allowed the Church to evaluate, in interpreting what we do know… Or we think we know…