Why did christianity take hold in the ancient world?

Because it was different from all other religions before, it stood out.All other regions (apart from Judaism) had more than one God, whereas Christianity like Judaism became a monotheistic religion (one god)It gave people an alternative,something new to believe/follow, and I agree what the other user says about oppression etc.

There is no prove and little evidence that Jesus existed because there is practically no historical account outside the bible. Bear in mind the gospels of Jesus where written 40 or so year after he supposedly existed so nobody that apparently knew him wrote these books, so there is no first had accounts therefore the 'real' story could have been made up/or exaggerated. Not forgetting the Jesus story is not original. There are many Gods with extreme similarities, so it's rational to think the divine part of Jesus story was influenced by other God myth stories.