I wrote the following in a Facebook dialogue with a friend. I won’t cite his words, but readers will get the gist of what he was arguing, from my response. He started by citing this passage in the Sermon on the Mount:
Matthew 5:22 (RSV) But I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, `You fool!’ shall be liable to the hell of fire.
I would point out that it is also legalistic when some folks make this an absolute. I wrote a post about this, showing that Jesus, Paul, and Peter used “fool” or “foolish” several times in Scripture. Paul said we were to imitate him, many times. We can’t say “never” and can’t say “always, at the drop of a hat.” It’s a balance, like most things in the Christian life.
Of course we’re not apostles and Jesus, but the point is that they are our models of behavior, and Jesus, especially, shows that His one saying wasn’t absolute, since He used the word “fool” in other contexts. Therefore, it can be done without sin.
“Fool” is mild, compared to the hyper-rhetoric used by many everyday, especially online. It’s used 135 times in the Old Testament as well: mostly by David and Solomon, in Psalms and Proverbs.
It’s all a balance. Sometimes a rebuke and strong language is called for; indeed, required (e.g., telling an alcoholic that he needs to stop drinking, or at least drinking and driving: not pleasant at all). Our task is to know when and how to do it.
I think the dangers lie at either extreme: making it an absolute, which it clearly isn’t (never say “fool”), or doing it all the time, as a matter of habit or ongoing uncharitable demeanor. The truth lies in the middle, and involves prudence and discretion.
It has to be based on objective truth, like a person going out drunk driving or blowing their whole paycheck on gambling, or never spending time with spouses or children. Those are objectively sinful, foolish things, and so we can say that “that person is acting like a fool.” That would be OT proverbial usage, and Pauline: Galatians 3:1: “O foolish Galatians! Who has bewitched you, before whose eyes Jesus Christ was publicly portrayed as crucified?” (RSV).
You argued that we can’t follow Jesus’ example because He was God, nor Paul’s, because he was an apostle, even though Scripture says that we should suffer like Christ, following his “example” (1 Peter 2:21), and Paul told us to imitate him (1 Cor 4:16; Phil 3:17; 2 Thess. 3:7-9), as he, in turn, imitated Christ:
1 Corinthians 11:1 (RSV) Be imitators of me, as I am of Christ.
1 Thessalonians 1:6 And you became imitators of us and of the Lord, . . .
I then brought up David and Solomon and the use of the word “fool” 135 times in the OT. Now you say their behavior (especially sexual) is too low; therefore, we can’t imitate them in that regard, either.
But sinners are all God has to work with. God said that David was a man after his own heart and made an eternal covenant with him, knowing from all eternity that he would commit adultery and have a woman’s husband killed in order to maintain that sin.
The Bible was largely written by murderers Moses and Paul, and also denier of Christ Peter and tax collector Matthew. The Psalms (inspired revelation, after all), mostly written by David, are central in the Daily Office, and recited at every Mass for the purpose of worship. So we’re already “imitating” David quite a bit; why not also in his use of the word “fool”?
I’m concerned about an overly legalistic, “super-pious” / “meek and mild Jesus” take of the teachings of Jesus, whereby no one could even engage in plain speaking. Jesus’ point in the Sermon on the Mount is to not engage in sinful judging of others, out of bitterness of pride or jealousy, anger, etc. The heart and interior disposition are always the focus in the Sermon on the Mount. We’re totally agreed on that. It doesn’t follow, however, that we can never say “fool”: as I have shown from several other Scriptures. Jesus uses the words “foolish man” in the same Sermon on the Mount:
Matthew 7:26 And every one who hears these words of mine and does not do them will be like a foolish man who built his house upon the sand;
So our disagreement is one of degree: can we virtually never (or literally never) call someone a “fool” or is there a permissible use?
It reminds me of people saying “you can never judge” (when we clearly have to judge in any number of circumstances), or saying that “turn the other cheek” is an absolute. It clearly isn’t, because Paul by no means turned his cheek during his own trial. He appealed to Roman citizenship to save himself from crucifixion, and even called himself a Pharisee, twice, for obvious dramatic effect.
In any event, we have to know how to apply a “judgmental” passage of St. Paul’s (one of many) such as the following:
2 Timothy 3:2-9 For men will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy,  inhuman, implacable, slanderers, profligates, fierce, haters of good,  treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God,  holding the form of religion but denying the power of it. Avoid such people.  For among them are those who make their way into households and capture weak women, burdened with sins and swayed by various impulses,  who will listen to anybody and can never arrive at a knowledge of the truth.  As Jannes and Jambres opposed Moses, so these men also oppose the truth, men of corrupt mind and counterfeit faith;  but they will not get very far, for their folly will be plain to all, as was that of those two men.
Now, how can we apply that passage (which entails a command: “avoid such people”), if we haven’t made a judgment as to certain people being habitual, extreme sinners and fools? Paul says, “their folly will be plain to all.” It’s typical Hebrew hyperbole, of course, but it means that many are able to see the folly. If we see it, we can also say that the persons committing it are fools. We make that judgment already, in order to be able to apply his command to “avoid such people.”
And how would we apply in real life, the “classic” passage in Proverbs?:
Proverbs 26:4-5 Answer not a fool according to his folly, lest you be like him yourself.  Answer a fool according to his folly, lest he be wise in his own eyes.
We can’t apply that practical wisdom without making a judgment beforehand that “Person X is a fool” and thus determine whether the situation calls for answering him or not. Proverbs was intended for practical wisdom to be utilized in life for all time, whether it was written by a sinner or not.
In fact, you are in effect (rightly) judging people who say “fool” too much, as fools, in this very post. You don’t use that terminology, but that’s what it plainly amounts to. You are arguing, in essence, “don’t act in this way (it’s folly and sin)!” And so you prove my point. You have made a judgment according to Scripture, that we are perfectly allowed to make. If you truly believed that you absolutely couldn’t judge anyone else or characterize any behavior as sinful and folly, and condemn it, you couldn’t even write this post. You would have to just sit back and say, “that’s for God to judge, not me.” But you’re “preaching” and saying (in effect), “calling people fools [excessively] is dead-wrong; don’t do it!”
And you’re right. It’s only a matter of working out details of how to apply the true maxim of Jesus. And that is fairly complex. The serious Christian disciple must exercise conscience, wisdom, discretion, timing, and prudence, guided by the Holy Spirit, in knowing what to do, when.
You’ve stimulated my mind, to make me think and write so much about this. That’s what I love about dialogue. Two views may seem more different than alike, but then upon examination in dialogue, they are seen to be fundamentally in agreement.
We totally agree if by “not calling people ‘fools’ ” you mean, not judging entire groups in bitterness and rage, which is what I think Jesus was driving at. It’s wrong to go around condemning entire groups (Democrats or Trump voters, black people, rich people, Protestants, Arabs, Mexicans, Irishmen, etc.): anyone different from us.
Thus, it’s wrong to say, “all Democrats are fools” or “all Trump voters are fools who have fundamentally compromised the Catholic faith.” That’s sinful judgmentalism. We’re called to believe the best of others (1 Corinthians 13), not the worst, and to esteem others higher than ourselves.
But it’s not wrong to say, “Harvey Weinstein is a fool” (just as Paul named and condemned Hymenaeus and Philetus [2 Tim 2:17] and Alexander the Coppersmith [1 Tim 1:20; 2 Tim 4:14] ), because that is a manifest truth: “fool” being basically a synonym for “habitual sinner” in Holy Scripture.
[see also additional lengthy dialogue on this topic on my Facebook page, with Dr. Dawn Eden Goldstein, from 12-27-17]