“For the law was given by Moses, but grace and truth came by Jesus Christ.”
John 1:17 KJV
The scripture is speaking of two different covenants (or ways of relating to God) revealed in this verse: Moses (the Old Covenant) and Jesus (the New Covenant). The word “but” divides this verse in two, showing us that “the law was given by Moses” and “grace and truth coming by Jesus Christ” are not the same thing. The Old Covenant was based on the law, which came through Moses to the Jews. The New Covenant is based on grace and truth, which came for all people through Jesus. Sadly, many Christians are ignorant of this truth and try to live their new covenant Christianity under old covenant principles. A good question to ask ourselves is: are we disciples of Moses or disciples of Jesus?
“He also told them a parable: “No one tears a piece from a new garment and puts it on an old garment. If he does, he will tear the new, and the piece from the new will not match the old. And no one puts new wine into old wineskins. If he does, the new wine will burst the skins and it will bespilled, and the skins will be destroyed. But new wine must be put into fresh wineskins.”
Luke 5:36-38 ESV
In Jesus’ day, if you were to sew a new piece of cloth onto an old garment to patch up a tear or a section which was worn out, it would shrink the first time it was washed and it would tear away from the old garment that had already shrunk. This would further damage the garment. This shows us that Jesus did not come to patch up the Old Covenant with the New, but instead, He came to replace it.
“For on the one hand, a former commandment is set aside because of its weakness and uselessness (for the law made nothing perfect); but on the other hand, a better hope is introduced, through which we draw near to God.”
Hebrews 7:18-19 ESV
You don’t put new wine (unfermented wine) into old wineskins. You’d need to use new wineskins because as the fermentation process occurs only new wineskins can accommodate the expansion of the gas. New wine in old wineskins would cause the old wineskins (which are already stretched) to burst and the wine would be lost. The Old Covenant law could never stretch enough to accommodate the New Covenant truths of mercy and grace.
You cannot mix law and grace and have a successful Christian life.