[excerpt]
For the first 280 years of Christian history, Christianity was banned by the Roman Empire, and Christians were terribly persecuted. This changed after the “conversion” of the Roman Emperor Constantine. Constantine provided religious toleration with the Edict of Milan in AD 313, effectively lifting the ban on Christianity. Later, in AD 325, Constantine called the Council of Nicea in an attempt to unify Christianity. Constantine envisioned Christianity as a religion that could unite the Roman Empire, which at that time was beginning to fragment and divide. While this may have seemed to be a positive development for the Christian church, the results were anything but positive. Just as Constantine refused to fully embrace the Christian faith, but continued many of his pagan beliefs and practices, so the Christian church that Constantine promoted was a mixture of true Christianity and Roman paganism.
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http://www.gotquestions.org/origin-Catholic-church.html
Yes, Constantine really did a number there, redefining and recreating a version of Christianity that suited his needs