What the Bible says about New Cloth on Old Garment
(From Forerunner Commentary)
Jesus' illustration derives from a well-known fact: No one with a reasonable amount of experience in mending clothes would waste a piece of new cloth to repair an old garment. If new cloth is used to patch an old garment, and the patch becomes wet, it shrinks as it dries and puts strain on the old garment. The tear becomes worse than it was.
Jesus is showing that His "new" doctrines do not match the old rites of the Pharisees, which required a lot of fasting. If His "new" doctrines were attached to their old ones, it would distort the truth. Christ is preaching against syncretism, the mixing of beliefs. We must completely replace the old human way of life with the new godly way of life (II Corinthians 5:17; Ephesians 4:22; Colossians 3:9-10). Because God's "new" way is righteous and spiritually strong, it cannot be combined with the "old" wicked and weak human way of life. They are incompatible.
Martin G. Collins
Parable of the Cloth and Wineskins
"No one puts a piece from a new garment on an old one; otherwise the new makes a tear, and also the piece that was taken out of the new does not match the old" (Luke 5:36). Mark's version of the same parable stresses that the "tear is made worse" when the new patch eventually "pulls away from the old" garment (Mark 2:21). Christ's message is clear: When it comes to doffing the old man and donning the new one, we cannot "mix and match." Successfully mixing them—combining them—is as impossible as serving two masters. We just cannot do it (Matthew 6:24)! The two men represent intrinsically and irreversibly opposing ways of life.
Charles Whitaker
Choosing the New Man (Part Three)