9 But when a dispute arises concerning matters of doctrine, and all take their weapons from the same Scriptures, of what weight will any one's life be able to prove? What then will be the good of his many austerities, when after such painful exercises, any one from the Priest's great unskillfulness in argument fall into heresy, and be cut off from the body of the Church, a misfortune which I have myself seen many suffering. Of what profit then will his patience be to him?
None; no more than there will be in a sound faith if the life is corrupt. Wherefore, for this reason more than for all others, it concerns him whose office it is to teach others, to be experienced in disputations of this kind. For though he himself stands safely, and is unhurt by the gainsayers, yet the simple multitude under his direction, when they see their leader defeated, and without any answer for the gainsayers, will be apt to lay the blame of his discomfiture not on his own weakness, but on the doctrines themselves, as though they were faulty; and so by reason of the inexperience of one, great numbers are brought to extreme ruin; for though they do not entirely go over to the adversary, yet they are forced to doubt about matters in which formerly they firmly believed, and those whom they used to approach with unswerving confidence, they are unable to hold to any longer steadfastly, but in consequence of their leader's defeat, so great a storm settles down upon their souls, that the mischief ends in their shipwreck altogether.
But how dire is the destruction, and how terrible the fire which such a leader brings upon his own wretched head for every soul which is thus lost, you will not need to learn from me, as you know all this perfectly. Is this then pride, is this vainglory in me, to be unwilling to be the cause of the destruction of so many souls? And of procuring for myself greater punishment in the world to come, than that which now awaits me there? Who would say so? Surely no one, unless he should wish to find fault where there is none, and to moralize over other men's calamities.
Source: On the Priesthood (New Advent)