The photogenic triumphal arch next to the Colosseum was erected by the Senate in a.d. 315 to honor Constantine’s defeat of the pagan Maxentius at the Battle of the Ponte Milvio (Milvian Bridge) Battle (a.d. 312). Many of the reliefs have nothing whatsoever to do with Constantine or his works, but they tell of the victories of earlier Antonine rulers.
Historically, the arch marks a period of great change in the history of Rome. Converted to Christianity by a vision on the eve of battle, Constantine ended the centuries-long persecution of the Christians, during which many followers of the new religion had been put to death in a gruesome manner. Although Constantine didn’t ban paganism (which survived officially until the closing of the temples more than half a century later), he embraced the Christian belief himself and began the inevitable development that culminated in the conquest of Rome by the Christian religion.