How did the roman emperor Constantine spread Christianity?

Constantine the Great did not spread Christianity. By the time of his reign, Christianity had already spread and had already become the religion of the masses in the Roman Empire, although there were still many pagans.

Constantine supported the Christians. He arbitrated between different doctrines of Christianity, promoted Christians in the imperial administration and built Christian Churches. The most notable of these churches were the original Basilica of St Peter's in Rome, the St John Lateran's Basilica (the city of Rome's first Cathedral and the original residence of the Popes), the Church of the Holy Apostles in Constantinople and Church of the Holy Sepulchre, in Jerusalem.