Luke 5:36-38 "No one tears a patch from a new garment to patch an old garment. Otherwise, the new garment would be ruined, and the new patch wouldn’t match the old garment. Nobody pours new wine into old wineskins. If they did, the new wine would burst the wineskins, the wine would spill, and the wineskins would be ruined. Instead, new wine must be put into new wineskins.”
The lesson of both parables is this: keep the new separate from the old, for in this way, both are preserved. Because the new wine would continue to ferment, it would stretch the old wineskins and they would burst. In the garment example, both the old and the new garment would be ruined because the new patch would have been torn from and ruined the new garment and it wouldn't match the old garment. In the parables, Jesus used "new wine" and "a new garment" to picture new ideas, new doctrines. Acts 17:19, "… May we know what this new doctrine…is?" The new doctrine Jesus taught was grace, not the law. "Jesus said, I am the way, the truth, and the life: no man cometh unto the Father, but by me," John 14:6. The Christian faith required that people realize they could not be righteous in themselves. They could only become righteous in God's eyes by having faith in Christ. Faith in Christ was the only condition whereby an individual would receive forgiveness and mercy. The doctrine of faith in Christ required a new wineskin.
Jesus was gathering out his church. Jesus was making an entirely new institution, which would not be a Jewish church, nor a Reformed Jewish church, but a wholly different institution, a Christian church. The Jewish system, based on the law of Moses, demanded absolute righteousness, which was impossible for sinners. Trying to combine Christianity with Judaism would have failed both because they are opposites: obedience to the law versus salvation through the grace and favor of Christ. The Jewish nation would have been convulsed and wrecked by the spirit of the new teachings. Moreover, the doctrines, themselves, would have gone down with the wreck of the nation. Using the language of the parable, the wineskin would have been "ruined" or "burst" (King James version) and the garment would have been "ruined," or "rent" (torn, KJV).
Why has God preserved both systems? Because each has a part of His eternal plan for all mankind, "to bless ALL the families of the earth." Genesis 22:18. Faithful Christians will live with Christ in heaven and will work with God's people, the nation of Israel. (Zech. 12:10, Jere. 31: 33, 34) The (then converted) nation of Israel will be the blesser nation to all other nations: “all nations shall flow unto it. And many people shall go and say, Come ye, and let us go up to the house of the God of Jacob; and He will teach us His paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem.” Isa. 2:2, 3 Ultimately, all mankind on earth will know and follow Christ in his righteous kingdom on earth.