Here's some of what I've read:
“Historians still marvel at the brilliance with which Emperor 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.’
‘Transmogrification,’ Langdon said. ‘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 miter, the altar, the doxology, and the communion, the act of ‘God-eating’—were taken directly from earlier pagan mystery religions.’
Teabing groaned. ‘Don’t get a symbologist started on Christian icons. 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. By the way, December 25 is also the birthday of Osiris, 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,’ Langdon said, ‘Christianity honored the Jewish Sabbath of Saturday, but Constantine shifted it to coincide with the pagan’s veneration day of the sun.’ He paused, grinning. ‘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," from pages 232- 233 of the Da Vinci Code by Dan Brown, published by Bantam.
The previous quote is in a fiction novel but is not fiction. A person can argue Church history all day long, but the facts remain. History shows that shortly after the death and resurrection of the Savior, the Roman Empire harshly persecuted the early believers. The more the faith was persecuted, the more it grew. The maltreatment continued until the time of Constantine the Great, who blended the true faith with pagan ideas and evil institutions. Most people are familiar with the story of how Constantine “the Great” conquered the known world with his Roman rule. During a religious experience, he reportedly saw the vision of a cross in the sky. Shortly after this, Constantine declared the entire empire “Christian” and stopped harassing believers. Instead, Constantine proved the old adage, “if you can’t beat ‘em, join ‘em.” And join them he did, but not without making many changes to the true faith.