You make the mistake of considering Christianity as something that developed over the course of a few years, from the death of Jesus to the time the gospels were written. But Christianity wasn't new. Only the name was new. Christianity was merely a stage in the meeting, cross-fertilization, metamorphosis of Western logic and Eastern mysticism. Look how the religion itself changed over the centuries, reinterpreting itself to meet changing times. Christianity is just a new name for a conglomeration of old myths and philosophies. All the gospels do is retell the sun myth and garble some of the ideas from the Greeks and Romans.