The Roman Emperor Constantine was actually the first amongst any kind of Roman leader to show support for Christianity, after centuries of feuding with the Christians.
The Romans truly hated Christians and Christianity for centuries, mostly because what the Christians believed just so happened to be in direct contrast to what the Romans believed, thus causing trouble (at one time, Christians were allowed to reside in Rome, though were shortly kicked out for various reasons). Also, the Christians believed no man could ever own or break the soul of another man, hence saying that being an "emperor" was pointless because he'll never reign over anyone. It was reasons like that which caused the Romans to hate the Christians.
However, Constantine came along and converted the Roman religion to Christianity around 306 A.D. About 70 years after this, Theodosius proclaimed Christianity at the main and sole religion of the Romans. Also, while Constantine reigned, he showed support by making Christians officials and giving them higher class jobs.
So, the Roman Emperor Constantine actively supported Christianity by converting the Roman Religion to Christianity around 306 A.D, as well as openly favoring Christians as officials.
Sorry for the lengthy-ness, I'm in AP classes including History, so I like to explain this stuff.
~Hope this helps m8