If I recall correctly, MacIntyre advances the thesis that Renaissance Aristotelians brought discredit to academic moral philosophy (and to Aristotelian moral philosophy in particular) with their claim that what they taught was sufficient by itself to make a student good. With a proper understanding of human nature (and of sin) we know that this claim is wrong.

Still, it is "natural" (even if fallacious) to judge a moral philosophy to be false if its teacher lived a rather immoral life. Or, we would think the teacher to lack any credibility and his teaching suspect.