"Judge not"

"THOSE who stand guilty of grievous sins should not judge those who are guilty of the same or lesser sins, as Chrysostom [Hom. xxiv] says on the words of Mat. 7:1, "Judge not." Above all does this hold when such sins are public, because there would be an occasion of scandal arising in the hearts of others. If however they are not public but hidden, and there be an urgent necessity for the judge to pronounce judgment, because it is his duty, he can reprove or judge with humility and fear. Hence Augustine says: "If we find that we are guilty of the same sin as another man, we should groan together with him, and invite him to strive against it together with us." And yet it is not through acting thus that a man condemns himself so as to deserve to be condemned once again, but when, in condemning another, he shows himself to be equally deserving of condemnation on account of another or a like sin."

~St. Thomas Aquinas: S.T., I-II, q. 60, a. 2, ad 3.