If a Christian’s attitude is right: Carson on judgmentalism

What does Jesus mean by his imperative in Matthew 7:1, “Do not judge, or you too will be judged”…?
This is not an easy area of one’s life to sort out. On the one hand, some people are so critical that they feast on roast preacher every Sunday lunch; and some preachers are so critical they level verbal barrages at most of their colleagues, especially those more fruitful than they. On the other hand, Jesus’ disciples ought to recognize some preachers as false because of their fruit (7:16), and dismiss them accordingly; the preacher who credits all his peers with precisely the same grace and insight falls far below Paul’s discriminating attitudes. The problem is that the Christian’s responsibility to discern, once granted, is readily warped into justification for harping criticism. The arch-critic is thoroughly at home with all the passages which encourage us to spot false prophets by their fruit. “I’m not being judgmental,” he protests, “I’m just a fruit inspector.” But by his own mouth, he stands condemned; he has become a fruit inspector, he has taken on himself some special role.
What is fundamentally at stake, I think, is attitude. This is clearly seen in that particular kind of critical spirit found in the gossip. It is not always the case that what the gossip says is malicious; what he says might, in fact, be strictly true. But it is always the case that he says it maliciously; that is, he speaks without any desire to build up, or any real concern to instill discernment. He wants only to puff himself up, or to be heard, or to enhance his own reputation, or to demean the person about whom he is speaking.
If a Christian’s attitude is right, provision is made for him to face another brother with his fault (see Matt. 18:15ff.). Indeed, spiritual leaders will not ignore open sin in one of their Christian brothers, but will try and restore him—gently, and aware of their own weakness (Gal. 6:1).
“Do not be judgmental,” Jesus says, and then adds, “or you too will be judged” (7:1). The latter clause may perhaps be taken like the first: if you are judgmental, others will be judgmental toward you. Alternatively, depending on the ambiguity of the Greek verb, the sentence may mean: do not be judgmental, or you will be condemned (whether by God or others). Either way, the clause adds stinging pungency to the injunction, and introduces the theological justification for abolishing all judgmental attitudes.
Adapted from D.A. Carson, Jesus’ Sermon on the Mount and His Confrontation with the World: An Exposition of Matthew 5-10, p. 106-107