When asked by John’s disciples as to why Jesus’ disciples did not fast, our Lord relates fasting to mourning and states that His disciples will “fast” after He has been “taken from them.” But then He goes on to speak of patching an old garment with a piece of new cloth and filling old wineskins with new wine, and these pictures don’t seem to fit.
Fasting was one of the formalities the Israelites under the Law Covenant kept to show their obedience to God. Actually, the Mosaic Law required only one fast per year, on the Day of Atonement, and those under the Law were bound to keep that fast.
However, sometimes public fasts were proclaimed, when the nation as a whole would ask God's forgiveness or beg for his help. Also, sometimes in time of need or grief, individuals privately fasted. However, by Jesus’ day it was apparent that many people looked upon fasting as a sign of piety, to be noticed by others and admired. Jesus spoke against this fasting “for show.” (Matthew 6:16-18)
But in Matthew 9:14-17 our Lord was making another point. One of Jesus’ teachings was that “all the Prophets and the Law prophesied until John” (Matthew 11:13). A new age was beginning. After centuries of trying (and failing) to keep the Law, a “new and living way” (Hebrews 10:20) was being opened to the Israelites. Jesus was about to die and pay the Ransom for Adam’s sin, thereby redeeming the whole world of mankind. But this new age meant an end to the Law Covenant and the beginning of the age of “grace.” This meant that justification to life was no longer to be sought by performing works or following formalities. Justification would come through faith in Jesus’ sacrifice.
Some of the Jews of that time perhaps wanted to put a “patch” on their faith by keeping the Law but accepting some of the teachings of Jesus. But this couldn’t make them whole. The “patched” garment wouldn’t justify them; they needed to completely “clothe [them]selves with the Lord Jesus Christ” (Romans 13:14).
Likewise the new “wine” was the doctrine of justification by faith in Jesus’ blood. Attempting to receive this new faith into a heart structured by belief in the Jewish Law would not be workable.
Complete faith in Jesus depended on the recipient recognizing that he/she could not keep the Law and therefore the Law was not able to give them eternal life. Each believer needed to accept Jesus as God’s gift of redemption replacing the demands of the Law.