Why does Paul say that judging another person condemns yourself?
Paul says judging another condemns yourself because the one who judges practices the same things, whether overtly or covertly in the thought processes. God's judgment is according to truth, and no one will escape it. Since every person out of Adam shares the same potential for sin, self-righteous judging exposes hypocrisy rather than righteousness.
Les teaches from Romans 2 that the moral man is inexcusable when he judges others. The person who points the finger at another forgets that when he points two or three that way, one or two are pointing back. Paul writes, "for wherein thou judgest another, you condemn yourself," and "for thou that judgest, doest the same things." Les explains that we may not do these things overtly, but what about covertly, or in the thinking processes? He says every one of us has the potential for all of this because we are out of Adam, and it is only the grace of God that keeps us from the same depths. God's judgment is according to truth against those who commit such things, even if only covertly, and no one will escape that judgment. Even the good person who would not dream of entering a homosexual relationship is just as guilty mentally in his thought processes. Jesus took the commandment "thou shalt not commit adultery" so far that none of us are clear of it; even if you think it in your heart, you are just as guilty as the person who does it.
Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.