In the roman empire it was illegal to be a christian. If you were a christian you would be persecuted. The persecution of christians made more people stand up for christianity.
Constantine was the "first christian emperor". He made it legal to be a christian, and christianity became the official religion of Rome. If you were non-christian you were persecuted.
The roman empire began to fall because they tried to solve all their problems with violence,violence cant rule forever. They forced people to do things they didnt want to. They were invaded by the Germanic tribes which created a downfall.