if the Mosaic law have not been changed, it would be still sinful to work on Sabbath. In one of the books of Moses there is a man who picks up wooden sticks on Sabbath, he is killed. However Jesus allows His 12 disciples to eat on Saturday, He also tells the lame to grab his bed and to walk on Sabbath which is in direct contradiction to this commandment. If the Mosaic law has not been changed, Jesus would not be able to sacrifice Himself on the cross because He is not from a tribe of Levi, only this tribe is allowed to bring sacrifices.
So the Mosaic law had to be changed but not the essence of the law, the essence of the law still remains. For why else would Jesus say in Matthew 5:38-39 38 You have heard it was said'eye for an eye, and tooth for a tooth.' 39But I tell you not to resist an evil person. If someone slaps you on your right cheek, turn to him the other also;…
As New Testament Christians in the New Covenant, The Mosaic Law of the Old Testament was written for the chosen people of Israel. Israel was chosen for a purpose—
Genesis 12:1-3. They were God’s instruments to accomplish a plan of rescue for the world: God becoming man as a member of the Jewish race and dying on the cross for man’s crimes against God. They were not chosen to be saved, rather they were chosen to be used to accomplish God’s plan of salvation. Many of those Jews took advantage of the mercy offered through that plan, and many went wayward. Israel’s being chosen did not secure their salvation. It secured a salvation plan for the world, and whether they individually participated in it or not was another question.
Christians are the chosen in the sense that those who are in Christ, putting their trust in Him, are the ones that benefited from that plan of salvation God was working out through Israel. We put our confidence in Him, so we are chosen regarding salvation.
God is the same yesterday, today, and forever. Jesus is the same yesterday, today, and forever because He is God. God doesn’t change, but that does not mean the way He deals with mankind doesn’t change. God dealt with Adam and Eve in one way, and He dealt with the Jews through Abraham with a different plan. He deals with Christians now through a new plan, a new covenant.