Over on her fine blog Startling the Day, my friend Elizabeth Hillgrove asked her readers to respond to a question in reference to Matthew 7:1-2 ("Stop judging, that you may not be judged. For as you judge, so will you be judged, and the measure with which you measure will be measured out to you" [NAB]). As part of my response, I noted that admonishing the sinner was traditionally considered one of the seven spiritual works of mercy.
For my non-Catholic audience, I should give a quick-and-dirty explanation: Catholics do not believe we're saved or justified by works alone. We're saved by the grace of God; we're redeemed by Christ's sacrifice on the Cross; faith allows us to be justified. Works of mercy must be understood as faith in action, or faith given physical expression. What we do as a consequence of our faith does have bearing on our judgment at the End of Days (cf. Mt 7:21; Rom 2:5-8; 2 Cor 5:10, 11:15; 1 Pet 1:17; Rev 20:12-13); as both St. Paul and St. James point out, a faith that doesn't translate into good works is spiritually empty, so much lip-service (1 Cor 13:2; Jas 2:14-26). For that reason, justification goes hand-in-hand with sanctification. (More on faith vs. works at Outside the Asylum.)
From The Catechism of the Catholic Church, paragraph 2447:
Traditionally, there are seven corporal works of mercy, i.e. acts done for another's physical good: 1) feed the hungry; 2) give drink to the thirsty; 3) clothe the naked; 4) harbor the harborless; 5) visit the sick; 6) visit the captive; and 7) bury the dead.The works of mercy are charitable actions by which we come to the aid of our neighbor in his spiritual and bodily necessities (cf. Isa 58:6-7; Heb 13:3). Instructing, advising, consoling, comforting are spiritual works of mercy, as are forgiving and bearing wrongs patiently. the corporal works of mercy consist especially in feeding the hungry, sheltering the homeless, clothing the naked, visiting the sick and imprisoned, and burying the dead (cf. Mt 25:31-46). Among all these, giving alms to the poor is one of the chief witnesses to fraternal charity: it is also a work of justice pleasing to God (cf. Tob 4:5-11; Sir 17:22; Mt 6:2-4):He who has two coats, let him share with him who has none and he who has food must do likewise (Lk 3:11). But give for alms those things which are within; and behold, everything is clean for you (Lk 11:41). If a brother or sister is ill-clad and in lack of daily food, and one of you says to them, "Go in peace, be warmed and filled," without giving them the things needed for the body, what does it profit (Jas 2:15-16; cf. 1 Jn 3:17)?
Likewise, there are seven spiritual works of mercy: 1) counsel the doubtful; 2) instruct the ignorant; 3) admonish the sinner; 4) comfort the sorrowful; 5) forgive all injuries; 6) bear wrongs patiently; and 7) pray for the living and the dead.
Now, of these acts, the one most likely to cause a fuss is "admonish the sinner": "Hey, wait a minute! Didn't Jesus tell us not to pass judgment on one another?" Yes, he did tell us not to judge one another. But saying "You did wrong and you should make it right" is not the same as "You're evil and you're going to Hell".
The focus of the passage in question—Matthew 7:1-5—is on self-righteousness, presuming moral superiority. When we offer correction, we do so humbly, in the knowledge that we're also sinners. As a practical matter, we must admonish children to teach them good behavior and punish lawbreakers to teach respect for the law. In this sense, Jesus was employing rabbinic exaggeration: he no more expected us to quit "judging others" in this sense than he expected us to literally tear out our eyes and chop off our hands to avoid sinning (Mt 5:29-30).
"Truly, I say to you, as you did it to one of the least of these my brethren, you did it to me. ... Truly, I say to you, as you did it not to one of the least of these, you did it not to me" (Mt 25:40, 45). With these words, Christ identified himself with every person that suffers, without regard to the victim's deserts or merits.
Our treatment of other people is now a reflection of our relationship with the Lord: "A tree is known by the fruit it bears" (Lk 6:22; cf. Mt 12:33). For the sake of that relationship, we must put aside our self-interest and "love our neighbor as ourselves" (Mt 22:34-40; cf. Mk 12:28-34, Lk 10:25-37).