When did emperor Constantine rule that Christianity will be the formal religion of the roman empire?

After witnessing an image of the crucified Jesus Christ in the sky, at least according to later Christian Propaganda, Emperor Constantine (who was Emperor of only one quarter of the then Roman Empire) proceeded to win the battle for Rome by defeating Emperor Maxentius in 312 C.E. It is as true, and rather more likely, that Constantine witnessed a falling meteorite which he was then told was a sign from the Christian God. Too, he may have come across a weather phenomenon know as the 'halo effect' by which the sun is reflected off freezing water in the sky to peculiar effect when viewed from the ground. It should be noted that at this time that the heavily outnumbered Constantine was desperate for any sign that would confirm his faltering, virginal faith. Constantine, Roman emperor from 306 AD to 337 AD, converted Christianity from a persecuted cult to a religion--one among many, however-- that was to be recognized throughout the Empire. In 312 AD, the emperor decreed that, from that time forth, the persecution of Christianity was to end. It is worth noting however, that Costantine was sufficiently canny to leave the heads of 'pagan' Roman gods on the coins, did not convert all the temples to churches, and did not himself get baptised until the hour of his death.