It goes back considerably farther than the Christian era. Many of the most central ideas in the gospels about Jesus, such as the "virgin birth" are found in much earlier civilizations going all the way back to Egypt and Sumeria. They were recycled religious ideas, brought into the time shortly following Jesus' death in order to more effectively market a new faith and make conversions. They would have been concepts much more familiar to the people of the time, and thus much more acceptable, than to a modern audience.