What does Mark 2:21 mean?
Anonymous persons have asked Jesus' disciples why they don't fast when John the Baptist's disciples and the Pharisees do. Jesus explains that it's all a matter of context. In addition to the one God-ordained fast (Leviticus 16:29–31) and the four additional corporate fasts (Zechariah 7:3–5; 8:19), the Pharisees fast twice a week, on Mondays and Thursdays, to practice discipline and express their piety. Jesus does not condemn their fasting, but does criticize the public spectacle they make (Matthew 6:16–18). Such a fast should be between themselves and God, not used to earn attention.Jesus compares the rigid, dry religion of the Pharisees to an old garment. As Jesus points out, He did not come to patch Judaism with a new piece of unshrunk cloth, because it would result in the destruction of the entire piece. He came to complete Judaism and renew it into what God intended. To wash the garment as white as snow (Psalm 51:7). If He "patches" Judaism, the patch will tear away from the old, leaving the Jews worse off than ever. Jesus comes to redeem, not plug holes.
The noun translated "tear" comes from the Greek root schisma. The verb form, schizo is used in Mark 15:38 to describe how "the curtain of the temple was torn in two, from top to bottom." The curtain separated the altar from the Holy of Holies where the Ark of the Covenant was once held. When Jesus is crucified, the curtain, or veil, tears, top to bottom, to show that the barrier between God and the people is no more. This same verb is used in Mark 1:10 when the sky splits open at Jesus' baptism and the Holy Spirit descends like a dove.
Jesus' relationship to Judaism isn't a patch that will tear away at first washing, ruining everything. When God splits something, it is to remove the barriers that keep His people from Him.