(2) Judgment is lawful in so far as it is an act of justice. Now it follows from what has been stated above (1, ad 1,3) that three conditions are requisite for a judgment to be an act of justice: first, that it proceed from the inclination of justice; secondly, that it come from one who is in authority; thirdly, that it be pronounced according to the right ruling of prudence. If any one of these be lacking, the judgment will be faulty and unlawful. First, when it is contrary to the rectitude of justice, and then it is called “perverted” or “unjust”: secondly, when a man judges about matters wherein he has no authority, and this is called judgment “by usurpation”: thirdly, when the reason lacks certainty, as when a man, without any solid motive, forms a judgment on some doubtful or hidden matter, and then it is called judgment by “suspicion” or “rash” judgment.
(2 ad 1) Thou shalt not judge. (Mt 7:1) In these words our Lord forbids rash judgment which is about the inward intention, or other uncertain things, as Augustine states. Or else He forbids judgment about Divine things, which we ought not to judge, but simply believe, since they are above us, as Hilary declares in his commentary on Mt. 5. Or again according to Chrysostom, He forbids the judgment which proceeds not from benevolence but from bitterness of heart.
*(**4) Now no man ought to despise or in any way injure another man without urgent cause: and, consequently, unless we have evident indications of a person’s wickedness, we ought to deem him good, by interpreting for the best whatever is doubtful about him…He who interprets doubtful matters for the best, may happen to be deceived more often than not; yet it is better to err frequently through thinking well of a wicked man, than to err less frequently through having an evil opinion of a good man, because in the latter case an injury is inflicted, but not in the former."
(Aquinas, ST II-II 60 - Judgment)
*I think the type of judgment we are most often condemned for making is the third type: rash judgment, where we condemn a person’s action based on our own perception of his motives for doing it. We are normally ignorant of what motivates a person and therefore we should not judge what we do not know.