Constantine was a lifelong pagan who was baptized on his deathbed, too weak to protest. In Constantine's day, Rome's offical religion was sun worship - the cult of Sol Invictus, or the Invincible Sun - and Constantine was its head priest. Unfortunately for him, a growing religious turmoil was gripping Rome. Three centuries after the crucifixion of Jesus Christ, Christ's followers had multiplied exponentially. Christians and pagans began warring, and the conflict grew to such proportions that it threatened to rend Rome in two. Constantine decided something had to be done. In 325 A.D., he decided to unify Rome under a single religion, Christianity.
Constantine was a very good businessman. He could see that Christianity was on the rise, and he simply backed the winning horse. Historians still marvel at the brilliance with which Constantine converted the sun-worshipping pagans to Christianity. By fusing pagan symbols, dates, and rituals into the growing Christian tradition, he created a kind of hybrid religion that was acceptable to both parties.
This is transmogrification, the vestiges of pagan religion in Christian symbology are undeniable. Egyptian sun disks became the halos of Catholic saints. Pictograms of Isis nursing her miraculously conceived son Horus became the blueprint for our modern images of the Virgin Mary nursing Baby Jesus. And virtually all the elements of the Catholic ritual - the mitre, the altar, the doxology, and communion, the act of "God-eating" - were taken directlly from earlier pagan mystery religions.
Nothing in Christianity is original. The pre-Christian God Mithras - called the Son of God and the Light of the World - was born on December 25, died, was buried in a rock tomb, and then resurrected in three days. December 25 is also the birthday of Osirius, Adonis, and Dionysus. The newborn Krishna was presented with gold, frankincense, and myrrh. Even Christianity's weekly holy day was stolen from the pagans.
Originally Christianity honored the Jewish Sabbath of Saturday, but Constantine shifted it to coincide with the pagan's veneration day of the sun.
To this day, most churchgoers attend services on Sunday morning with no idea that they are there on account of the pagan sun god's weekly tribute - Sunday.