When the Pharisees complained against Christ's disciples, our Lord Jesus said, "No one puts new wine into old wineskins, or the new wine will burst the skins, and will spill out, and the skins will be ruined." (Luke 5:37) Christ's analogy describes the problem of trying to make the New Covenant fit within the Old Covenant of Law. You will loose the New, and destroy the Old in the process. Since the apostles' time, men have tried to mix the Law and Grace. But the New Covenant will not fit inside the constraints of the Old.
Remember Peter's vision in Acts chapter 10 where a sheet came down from heaven, containing both clean and unclean animals. A Voice told Peter to kill and eat, and Peter argued with Him saying, "No Lord, I've never eaten anything unclean or unholy." Then the Voice spoke to him again saying, "What God has cleansed, no longer consider unholy." This happened three times, leaving Peter confused as to why God would tell him to violate the Old Covenant law.