Reason for Corruption?

One possible reason for corruption of the patristic church was the influence of outside religions, particularly the idol-heavy pagan religions:

C. Guignebert, in an illuminating article, [FN 59] points out that for the first five centuries many converts from paganism to Christianity lived a sort of double religious life, which made them what he calls demi-Christians. Among the reasons he gives for this situation are syncretism, poor instruction in the faith, and the scandal of Christian converts who lapsed back into either partial or total paganism.

[FN 59: "Les Demi-chrétiens" 65-102.]

From Fathers of the Church series (vol. 68), St. John Chrysostom, Discourses Against Judaizing Christians, translated by Paul W. Harkins, p. xxxiv.