Christianity will continue to grow, although it will have a tendency to fragment without a Council of Nicaea. Relations between these various branches of the Church will be far more cordial than OTL though, apart from the seriously far out groups, who'll be disliked. There might be inter-communal violence between Christian groups, but nothing on a particuarly large scale.
The Empire itself won't see much of a return to the old traditional style of worship- it was moving towards monotheism of some description long before Constantine. Perhaps we could see Roman religion being understood as something like Hinduism, with all of the old Gods being viewed as various parts of a single divinity, as represented by the Unconquered Sun.
Outside the Empire, Christianity may very well become the state religion of Sassanid Iran. The Sassanids themselves may hold some sort of Council of Nicaea, and start persecuting heretics. Which could be fun. Christianity spreading east from Iran into India, whilst remaining only a minority community in Europe. Any form of Iranian Christianity will probably be rather dualistic, since it'll be open to a large amount of Zoroastrian influence.
Hope this helps!