“Your moral challenge to my behavior is invalid, because Jesus once said ‘Judge not.'”
With a single quotation, Christians are thus forbidden from declaring truths authoritatively on morality. This is sometimes what is honestly meant when this quotation is used.
What is also implied is that Christians must also first prove they are morally perfect before declaring moral truths, and any assumed moral imperfection means the moral declaration is automatically invalid. Because the Crusades.
The relevant verses are found in two places:
[Mat 7:1-5] 1 “Judge not, that you be not judged. 2 For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. 3 Why do you see the speck that is in your brother’s eye, but do not notice the log that is in your own eye? 4 Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? 5 You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother’s eye.
[Luk 6:37-38] 37 “Judge not, and you will not be judged; condemn not, and you will not be condemned; forgive, and you will be forgiven; 38 give, and it will be given to you. Good measure, pressed down, shaken together, running over, will be put into your lap. For with the measure you use it will be measured back to you.”
These are referring to judging the character of another person or making public determinations of a person. They have nothing to do with judging matters or issues. It is clear that we are indeed to “judge” moral matters of importance:
[Luk 12:57-58] 57 “And why do you not judge for yourselves what is right? 58 As you go with your accuser before the magistrate, make an effort to settle with him on the way, lest he drag you to the judge, and the judge hand you over to the officer, and the officer put you in prison.
[Jhn 7:24] 24 Do not judge by appearances, but judge with right judgment.”
Clarity comes here when we understand the distinction in judging is twofold; one refers to having authority to punish (which we have or do not have in accordance with our station/authority), and one refers to judging a matter, which we must do, and are commanded to do:
[1Co 6:1-6] 1 When one of you has a grievance against another, does he dare go to law before the unrighteous instead of the saints? 2 Or do you not know that the saints will judge the world? And if the world is to be judged by you, are you incompetent to try trivial cases? 3 Do you not know that we are to judge angels? How much more, then, matters pertaining to this life! 4 So if you have such cases, why do you lay them before those who have no standing in the church? 5 I say this to your shame. Can it be that there is no one among you wise enough to settle a dispute between the brothers, 6 but brother goes to law against brother, and that before unbelievers?
Therefore, Jesus’ words are cautionary; firstly, avoid judging matters which you are not involved in and let the sanction of proper authority have its sway. Second, when you judge a matter, first judge your own failings in the same area to be fit to judge what you see (lest you judge it improperly and thus create incentive to ignore your own failings). Third, do so with the intention of restoring the person to holiness, not merely proving someone else is in the wrong:
[Gal 6:1-2] 1 Brothers, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness. Keep watch on yourself, lest you too be tempted. 2 Bear one another’s burdens, and so fulfill the law of Christ.