Prior to Christianity become a 'state-accepted-sponsored' religion, it was essentially 'pure' in belief and practice by those who followed the teachings of Christ.
As soon as it became accepted and 'promoted' by the state, many people of wealth and power 'became Christians', just to curry favor with the new 'fashionable' religion that was favored by the Emperor. Consequently, earthly and worldly considerations (power / property / wealth) started becoming more important for those coming into the 'new' religion of "Christianity", obscuring the real message of 'love and peace and faith in Christ'. The new "Kings" and the "Pope" became engulfed in worldly and earthly power over their subjects (see 'Borgias').
Certainly the church gained a lot of temporal power and wealth. You would expect that for a group that goes from being hunted down and killed by the state, to a group that enjoys the favor of the emperor. But where's the evidence for some significant change in doctrine or even practice?