Matthew 9:17

Neither do men put new wine into old wineskins: else the skins burst, and the wine is lost, and the skins perish: but they put new wine into fresh wineskins, and both are preserved.

This illustration refers to the custom of putting wine into the bladders and skins of animals. Only new wineskins could serve for unfermented or new wine. After fermentation, the skins hardened and became brittle, thus becoming entirely unsuitable for new wine, yet continuing to serve well enough as containers for old wine. The application was that Christ did not pour the new wine of his teaching into the old wineskins (John's disciples) but into new wineskins (his disciples). It is noteworthy that none of Jesus' disciples had previously been followers of John. The reason is explained in this passage. It took new hearts, fresh viewpoints, unaccustomed to the practices and prejudices of old ways, to contain the marvelous new teachings of Christ.