Constantine was the emperor and did much to help solidify Christianity’s position. He gave Imperial monies for the printing of bibles and the building of churches, for example. He gave Christianity a prominent spot light and he made being a pagan difficult. This trend was – for the most part – continued by the emperors after him.
The one key exception was Juilan, also known as Julian the Apostate. Julian was raised as nominal Christian but secretly was a pagan. When he came to power, he came out of the closet, as it were. He desired a pagan revival and reinstituted pagan festivals. He made it illegal for Christians to teach the classics, saying, “Let them teach Matthew or Luke in their churches”. He brought back exiles – such as Athanasius - in hopes to create dissension. He did nothing to punish crimes by pagan mobs against Christians. He even wrote a book Against the Galileans against Christians. Yet he only reigned about 18 months and was killed in battle. His efforts failed; he was paganism’s “last hope”.
After Julian was Theodosius, who actually made Christianity the official religion of the Roman Empire. Theodosius also made paganism illegal and persecuted heretics. He supported the Council of Constantinople in 381 AD, which resulted in the final version of what we now call the Nicene Creed and which became a standard for orthodox theology.