After two chapters describing the citizens of His kingdom, Jesus turns the whole sermon back on us. The question is no longer whether we admire His teaching but whether we will do it. And notice where the doing starts. Not out there with the world that needs fixing, not with the brother who needs straightening out, but in the mirror. The first act of obedience in Matthew 7 is to look inward and deal honestly with our own sin.
This verse does not forbid all evaluation. Just a few verses later Jesus tells us to watch for false prophets and inspect their fruit, and in John 7:24 He commands us to judge with right judgment. What He forbids is the proud, habitual fault-finding that sets itself above everyone else. The Greek points to something ongoing, a default setting of criticism rather than an occasional honest assessment. Jesus is not after the courtroom; He is after the attitude of the heart.
Most of us would rather start with the window. It is easier to scan the room and score everyone in it than to turn the lamp on our own hearts. But discipleship that skips the mirror is not discipleship at all. It is performance.