Then the disciples of John came to him, saying, “Why do we and the Pharisees fast, but your disciples do not fast?” And Jesus said to them, “Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast. 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.” (Matthew 9:14-17)
This passage should be looked at within the context of Matthew 9:13, which reads, "Go and learn what this means, 'I desire mercy, and not sacrifice.'" That verse itself is in the context of salvation. Only those who know they are sinners in need of a savior desire Jesus and call out to him. Everyone is a sinner, but many think they are good, because they compare themselves to other people and not God himself. They refuse Jesus, because they don't need him. They are not sick. They do not need the doctor. The sinner does. God only saves bad people. Everyone is bad, but most think they are good. People who know they are bad cry out to him for salvation. He is ready and willing to save everyone who calls out to him. Faith is that true belief that you are a bad person who needs salvation from a good God.
So what does all this have to do with fasting and wineskins and patches on clothes? Well, when Christ says, "I desire mercy, and not sacrifice," he is saying that he desires a sinner to cry out to him for mercy, like I wrote above, and not someone who goes through some outward ceremony to garner God's appreciation. God is a saving God, not a pagan deity that needs to be appeased with offerings. Fasting is one such ceremony.
Does this mean we are not to fast? By no means, but like all sacrificial/ceremonial actions--baptism, communion, fasting, even coming to church--they are performed by the born-again Christian out of gratitude for his or her salvation. This takes time to build up, and is only done as the Spirit inspires one. I think this is crucial to understand, because this is something that is done in the church today. You're a Christian now? Here are all the duties that come with the title, here you go! And then all of this crushing sacrificial/ceremonial stuff is laid upon the new believer as new burden. But Christ said his yoke is easy an his burden is light. When one becomes a Christian, the Spirit is working within him and brings forth the desire for sacrifice. Sacrifice in no way leads to salvation, but God showing mercy does, and sacrifice only comes after salvation as an urge within the Christian to please God for saving him. So in Acts 8, Philip brings the Ethiopian Eunuch to Christ and then this happens:
And as they were going along the road they came to some water, and the eunuch said, “See, here is water! What prevents me from being baptized?” And he commanded the chariot to stop, and they both went down into the water, Philip and the eunuch, and he baptized him. (Acts 8:36-37)
Upon being converted, the desire to be baptized is built up inside by the Holy Spirit. The desire to congregate with other Christians begins to arise, too, as well as to partake in communion. Now, fasting is an act of repentance, and that may build up more and more as the Christian wrestles with sin. However, a new Christian, someone who has just come to Christ, does not usually have the desire to fast, and Jesus knows this, hence our passage (Matthew 9:14-17).
The disciples of John fasted, and indeed John's was a baptism of repentance, of preparation for Christ, essentially of the Old Covenant. It is still at heart a ceremonial conversion, and faith in Christ is still needed--the only thing that saves. John himself says this to his disciples in John 3. I'll reprint the whole thing here:
Now a discussion arose between some of John's disciples and a Jew over purification. And they came to John and said to him, “Rabbi, he who was with you across the Jordan, to whom you bore witness—look, he is baptizing, and all are going to him.” John answered, “A person cannot receive even one thing unless it is given him from heaven. You yourselves bear me witness, that I said, ‘I am not the Christ, but I have been sent before him.’ The one who has the bride is the bridegroom. The friend of the bridegroom, who stands and hears him, rejoices greatly at the bridegroom's voice. Therefore this joy of mine is now complete. He must increase, but I must decrease.”
He who comes from above is above all. He who is of the earth belongs to the earth and speaks in an earthly way. He who comes from heaven is above all. He bears witness to what he has seen and heard, yet no one receives his testimony. Whoever receives his testimony sets his seal to this, that God is true. For he whom God has sent utters the words of God, for he gives the Spirit without measure. The Father loves the Son and has given all things into his hand. Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him. (John 3:25-36)
John is telling his disciples, in short, the ceremonial/sacrificial practices of the Old Covenant are meaningless without faith in the one the ceremonies point to. Now that Christ is here, we all need to put our faith in him. Remember, the Old Testament salvation was by faith, too, but it was faith in Christ to come. We showed we had faith by ceremonies and sacrifices, but now that Christ is come, our faith is in the Christ of the Bible, explained in the pages of scripture. He himself has performed the ceremonies and sacrifices, and so all we need is faith without ceremony. Now, the Holy Spirit will compel us gradually to offer insufficient sacrifices out of gratitude, but they are not the same as Old Testament sacrifices. As John says above, all that matters is faith. "Whoever believes in the Son has eternal life." Now, what comes after salvation is obedience out of gratitude, as the Spirit compels us.
Back to our Matthew passage, Jesus is saying the same thing. Now that Jesus has come, repentance is not a precondition of salvation. This is a big sticking point in the church today, and books like the Marrow of Modern Divinity have been written to point out the distinction (and have been condemned by legalists as a result). Now that the bridegroom is here, Jesus says, faith alone saves you. There is no precondition to salvation. Now, once you are saved, the Spirit will compel you to repent, and sometimes fast, but that is not a precondition to salvation. What Jesus is speaking out against are the people who are applying preconditions to salvation. In Galatians, Paul is writing to a church that was saved through faith, but then Judaizers came afterward and told this church that faith was not enough and that they needed to add ceremony--not as the Spirit compelled them but as a precondition to true salvation. Your faith wasn't enough, they told the Galatian church. Paul had to set them straight with the gospel again.
So, Jesus uses a couple of comparisons to show that when Old Covenant ceremony is yoked onto a new believer, it will quench the Spirit and crush their faith away, likewise if people of the Old Covenant say that new believers are now part of their covenant. No, the Old Covenant people need to be converted to the New Covenant now, not the other way around! The New Covenant is that of faith alone in Christ alone.
All of this should be encouraging to the new believer, for the good news of Christ is that salvation is by faith alone in Christ alone. Never let anyone tell you that you are "doing it wrong." If you have an abiding faith in Christ, the Spirit will compel you to make sacrifices at the rate and speed that God knows you can handle. Don't try to overdo your new life in Christ. Recenter yourself with the Gospel as given to us in God's Word, and the Spirit will build a Christian life upon your faith. Don't worry! Eventually, you will be undergoing sacrifices, trials, and tribulations--and great blessings--like the rest of us!