Jesus told this parable: "No one puts a piece of unshrunk cloth on an old garment, for the patch tears away from the garment, and a worse tear is made. Neither is new wine put into old wineskins. If it is, the skins burst and the wine is spilled and the skins are destroyed. But new wine is put into fresh wineskins, and so both are preserved." (Mat 9:16–17, Mark 2:21–22, Luk 5:36–39).
When new wine is put into new wine skins, the wine produces gas and the new
wine skins expands (Job 32:19). Since only new skins expands, you can't use old skins for new wine. The same goes for fabric, only new fabric shrinks.
If one fast for the wrong reason or at the wrong occasion (e.g. at weddings),
it would only be done to gain attention and admiration (Mat 9:14–15,
Mark 2:18–20, Luk 5:33–35). It can be compared to pouring new wine into old wine bags or sewing a new patch of unshrunk fabric onto an old, well-washed garment.
However, the main meaning of this parable is that we can't merge the Old Covenant that has the Mosaic Laws, with the New Covenant that Jesus has introduced
(Heb 8:8–13, Jer 31:31–34, Rom 7:6, 2 Cor 3:6), see Jesus and the Law.
For information on other parables, see Jesus' Parables.
