Which emperor of Rome, around 300 AD, was first to become a Christian?

The pivotal moment for the Roman Empire's religious landscape occurred in 312 AD, on the eve of the Battle of the Milvian Bridge. According to tradition, the future emperor had a vision of a Christian symbol in the sky—either a cross of light or the Chi-Rho symbol—and heard a voice declare, "In this sign, you will conquer." He ordered his soldiers to paint the symbol on their shields, and after winning the decisive battle, he attributed his victory to the power of the Christian God.

Following his victory, he and his co-emperor Licinius issued the Edict of Milan in 313 AD. This landmark decree officially ended the state-sanctioned persecution of Christians that had raged under previous emperors like Diocletian. The edict granted religious tolerance throughout the empire, allowing Christians to worship freely and ordering the return of their confiscated property. This act fundamentally changed the status of Christianity from a persecuted sect to a legitimate, and soon favored, religion.

While he championed the faith for most of his reign, convening the Council of Nicaea and funding the construction of churches, he himself was not formally baptized until he was on his deathbed in 337 AD. This was a common practice at the time, as many believed that baptism washed away all prior sins, and waiting until the end ensured a clean slate before entering the afterlife. His conversion and patronage paved the way for Christianity to become the dominant religion of the Western world.