The Apostle Paul in Galatians is the angriest he is in any of his letters, he omits the giving of thanks and laynches straight into his argument - that the Galatians are in danger of falling away from the doctrine of grace because of the Judaizers' mixing up the Law and the Gospel in their preaching. He has nothing to be thankful for! But in Philippians 1.15 Paul speaks of those who "Preach Christ even from envy and strife," and goes on to say how they "Preach Christ from selfish ambition, not sincerely, hoping to add affliction to my chains." These were insincire Gospel ministers! So what does he say of them? "What then? Only that in every way, whether in pretence or in truth, Christ is preached; and in this I rejoice, yes, and will rejoice." Here were these wicked men who weren't preaching the Gospel because they believed in it, but to make Paul's life more difficult, and he says, "I rejoice," because Christ is being preached.
But in Galatians he says, "But even if we, or an angel from heaven, preach any other gospel to you than that which we have preached to you, let him be accursed."
Take these two passages together, and we find that it was Paul's principle - and ought to be ours - that we are not to judge the man, but the message. It has happened that men who once taught the truth have become heretics - the Bible commentator George Bush (no relation) became a Swedenborgian minister, but the great question is not, what did he become, but rather, what do his books teach! At the same time, after he became a Swedenborgian, the fact that he was a respected Bible teacher did not make the heresy he taught right! Not the man, but the message.
The personal character of a minister is important, but it does not necessarily affect his message. The King James Bible was translated by a group of Anglican ministers and one layman. One of the translators was a drunkard, others were involved in persecuting God's people, even to death! King James himself, who commissioned it, held drunken parties at his palaces (in mitigation, this was partly because his father-in-law was a very heavy drinker, and thus any party he went to involved a lot of alcohol). Does this matter? Only if it can be shown to have affected the text! In fact the AV does an excellent job of faithfully rendering warnings about drunkenness. Not the men, but the message.
So no expose of a teacher that majors on personal things is enough to prove that we ought not to be listening to him. On the other hand, many (not all) heretics have led blameless lives - this is one of the ways they draw people in! Again we say, Not the man but the message. Of course, sinners that we are, we all like a little bit of prurient gossip, but if all we can say against a man is that he had a less than wonderful personal life, we may be able to say that he is unfit to teach, but we have not proven that he is a false teacher.
Not the man, but the message.