I asked this over on /r/AskHistory but I was looking for more scholarly answers. So I thought I'd post the question here.
I'm reading through the fictional series The Warlord Chronicles by Bernard Cornwell, which is a take on the Arthurian mythology with a more realistic bent. It takes place during the 6th century and has an interesting take on the tensions between Paganism (Britannic druids) and Christianity during the post-Roman period.
In it the Christians are pretty militant in demanding that there is only one god (their god) while the pagans openly accept polytheism. This got me thinking about when and how Christianity became so militant against other religions.
So when did Christianity become focused on conversion and the denial of other gods & religions? Did this come from Judaism? Or was this a Roman influenced phenomenon?
Monotheistic religions generally are less tolerant of paganism or even other monotheistic religions than polytheistic religions. Generally the rationale was that because there is only one God, the deities of polytheistic religions are demons or deceivers of some kind. Your question is rather vague. Are you talking about Christianity specifically in old Britain? Christianity deals with pagan traditions differently in different parts of the world, often incorporating aspects of pagan culture. For example, in Central America pagan beliefs about blood sacrifice have been incorporated into contemporary Christian imagery of the Crucifixtion.