In Matthew 7:5 Jesus uses the title "hypocrite." He has used it before... but this is different.
The context seems clear enough. "Don't condemn others and knit-pick at their righteousness, when you've got your own to worry about. That is, don't say one thing and not apply it to yourself." Something I didn't realize however (until R. T. France pointed it out) was that the title, hypocrite, that Jesus applies to disciples who act this way is elsewhere almost exclusively used of Jesus' opponents. That is, it's not normal for him to call his disciples hypocrites (remember, here he is primarily teaching his disciples). Normally he more or less says "Disciples of mine, don't be like the hypocrites..." but here it's more like "You actually are being hypocrites!" It's the difference between warning them of the danger of becoming like the Scribes and Pharisees, versus Jesus actually spotting hypocrisy in them. And where of all places do we find it? In judging one another.
It seems that our Lord, many times, seeks to elevate forgiveness, and mercy, and sympathy toward others above just about everything else. He says forgive or you won't be forgiven. He says if you condemn your brothers, you'll be condemned.
This verse had particular bearing on me, as I was contemplating rebuking a brother (just slightly) but upon further contemplation reasoned that it wouldn't be best. My flesh wanted to rebuke, not the Spirit of God in me, I think.
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Therefore, because Jesus seems unrelenting in using a strong term like hypocrite even toward his own disciples on this issue, it would seem that one of the quickest ways to make yourself an opponent of God (and to be unChrist-like and undisciple-like) is to go around condemning and knit-picking others' relationship with God.