From the BBC: Did a meteor over central Italy in AD 312 change the course of Roman and Christian history?
A team of geologists believes it has found the incoming space rock's impact crater, and dating suggests its formation coincided with the celestial vision said to have converted a future Roman emperor to Christianity.
It was just before a decisive battle for control of Rome and the empire that Constantine saw a blazing light cross the sky and attributed his subsequent victory to divine help from a Christian God.
Constantine went on to consolidate his grip on power and ordered that persecution of Christians cease and their religion receive official status. [continue]
Related links:
Constantine I of the Roman Empire -from Wikipedia
Constantine the Great - from the Catholic Encyclopedia
Battle of Milvian Bridge - from Wikipedia
Labarum (Chi-Rho) ("Labarum is the name by which the military standard adopted by Constantine the Great after his celebrated vision (Lactantius, "De mortibus persecutorum", 44), was known in antiquity.")