Answer: 3. Christianity became legal to practice.
In the 4th century, Roman emperor Constantine accepted Christianity, and the Edict of Milan was birthed in 313. This edict stated specifically that practicing Christianity was now legal, and it led to the massive spread throughout Rome. This massive spread was especially made possible as a result of the edict which also allowed for freedom of worship throughout the Roman Empire.
The acceptance of Christianity by Constantine did not affect the belief system of Christians in any way, neither did it become the official religion.