“Nor do they put new wine into old wineskins, or else the wineskins break, the wine is spilled, and the wineskins are ruined. But they put new wine into new wineskins, and both are preserved” (Matthew 9:17 NKJV)

Jesus gave this response to the disciples of John the Baptist when they asked why Christ’s disciples didn’t fast. To understand His response we must first understand the metaphor He used. In that day containers for liquid were often made from animal skins. In the fermentation process of new wine, yeast converts the natural sugar in the grapes into alcohol and CO2. This causes expansion. New wineskins can handle this expansion because of their flexibility. On the other hand, old wine has finished its fermentation process and old wineskins have aged and lost their flexibility.
In context, Jesus uses this metaphor to compare old wine to the Old Covenant of the Law and new wine to the New Covenant of Grace that He was inaugurating. The hearers then, are compared to the wineskins. Some, will be unable to understand Christ’s redemption and will continue to pursue good works and ritualistic religion as a means to please God. They are like the old wineskins. Yet, others will recognize their own sinfulness and rely on Christ’s sacrifice and grace. They are like the new wineskins.
We have to be willing to let go of our own human effort at righteousness and freely receive Christ’s sacrifice for our sin in order to receive this “new wine,” this new covenant with God.