Constantine was raised pagan and was pagan most of his life. He was an avid sun worshipper. However, he nominally accepted Christianity and was baptized just before he died. But Christianity did not replace paganism in his empire, but rather, Christianity adopted paganism. Christians were persecuted before Constantine accepted Christianity but afterwards, it was cool. But the problem is the form of Christianity that followed.
Constantine converted sun-worshipping pagans to Christianity by fusing pagan symbols, rituals and dates into a hybrid religion. Egyptian sun disks became the halos of Catholic saints. Remember the Greek sun god helios....hence halo....and sol in Roman mythology...or sol invictus...the "unconquerable sun." Images of Isis nursing her son Horus were recast as Mary and baby Jesus. The Persian sun god Mithra was said to have been born on Dec. 25..."natalis invicti"...rebirth of the winter sun, unconquered by the rigors of the season. That of course was the obvious choice for Christmas even though there's no biblical evidence that we as Christians should commemorate the birth of Jesus.
Sunday (first day of the week) was also a very holy day to pagan sun worshipping cultures. Where as throughout the entire bible, God's people observed the seventh day of the week as His holy day. But Constantine, in an Edict back in A.D. 321 declared Sunday as the official day of rest on which markets were banned and public offices were closed (contrary to Ex. 20:8-11, Neh. 13:15-21). Furthermore...a few years later at the Council of Laodicea, they outlawed the observance of the seventh-day sabbath: "Christians should not Judaize and should not be idle on the Sabbath, but should work on that day; they should, however, particularly reverence the Lord’s day and, if possible, not work on it, because they were Christians" (Canon 29 [A.D. 360]).
After Constantine "accepted" Christianity and amalgamated it with paganism, the persecution of Christians did not cease. Christians that kept the true faith and rejected this spurious new faith continued to give themselves up as martyrs who loved not their lives unto death.
Throughout the bible, God's relationship with his church is likened to that of a marriage (See story of Hosea and Gomer, parable of the 10 virgins, etc). Yet in Revelation, God talks about a church that has played the harlot...that has committed spiritual adultery against God. That's why some pagans laugh at most strains of Christianity. I saw a bumper sticker one time that said something about Christianity having pagan footprints.
