Answer: Christianity gave hope to oppressed people.
Step-by-step explanation:
The Roman Empire had been a very unequal empire. Romans were at the top and looked down on other nationalities with the Roman Citizenship being a treasure at some point. Even amongst the Romans themselves there were social classes. From the Emperor to the High Families to the Senators and so on. They were treated differently according to their social class.
Then Christianity came, a religion of Equality. A religion that said that God didn't care for what profession you did or which family you were born in, God cared for you all the same. The Christians educated their members and took care of Widows. They opened soup kitchens and shared food and they cared for each other as Jesus said they should. The gave hope to the Oppressed here on Earth and for their souls afterwards. On Earth they had brethren and did not feel alone and they also had the hope that when they died they had a place in heaven regardless of their status on Earth.
Many Early Romans were drawn to Christianity therefore and their numbers kept rising regardless of the persecutions various emperors like Diocletian meted out on them till Constantine the Great saw the benefits of Christianity in the state and legalized it.