Christians must practice Judaism as promulgated by Jesus
For argument's sake, every Christian has become a Jew because we are one in Jesus Christ, a Jew.
In Matthew 7:21; 12:50, Jesus says that the one who will be saved is the one who does the will of His Father in Heaven.
Where do we find the will of the Father? In the records of the Assyrians, of the Babylonians, of the Greeks, of the Romans? No! The will of God was given to Moses and codified in the Scriptures that Jesus refers to as “the Law and the Prophets". If it is irrevocably so, does not the source of our Christian belief and code of living makes us a Jew even though not by race? Yes, indeed. If we do the will of the Father, Jews and Gentiles, we become a brother, a sister, and mother according to Matthew 12:50. As such, we become a Jew since Jesus is a Jew.
Which obedient child of the Father would exclude oneself to not do His will claiming that only the Jew was required to obey such and such commandment? As we read, Jesus did not single out any ethnicity that would be exempt from doing the will of the Father, since those who will enter the Kingdom must do the will of God, whether you are a Jew or any Gentile.
Fast forward to Jesus and to Roman 10:12 and Colossians 3:11, the statements in those verses do not imply that the Jews no longer exist as a race or a nation. If there is no longer a distinction between the ethnicities, that means that every single human being is required to live His strict code of life which was for the Jews but now is extended to all Gentiles. We, Gentiles, did not have our own Moses through whom God transmitted His code of living. We, Gentiles, did not have our own prophets to call attention to violations of God’s code of living. When God sent His Apostles to gather the Gentiles under one umbrella with the Jews, He did not give them a separate code of living. Since Jesus fulfilled the Law, the Jews and the Gentiles are no longer required to follow the law that has to do with the Temple services or animal sacrifices. When the question of circumcision of the Gentiles was discussed, the Apostles agreed not to have them circumcised, as Paul echoes the reasoning in Romans 2:28-29.
When Christians disagree on a fundamental doctrine like the correct day of worship, whether it is on Saturday or Sunday, those who choose Sunday use what they consider the strongest argument against the Jewish Sabbath day: the sabbath day was given to the Jews only. However, we can infer from those verses that there is nothing that was required of the Jews that constitutes an exemption for a Christian who seeks out to please God. We are all called upon by Jesus to do the will of God as He does, Jews and Gentiles. That is nothing new. Long ago, in Isaiah 56:3-7, Jehovah invited all Gentiles to Judaism.
While, we, pagans, did our things as we pleased, God still commanded the Jews to follow a strict code of living. When the Jews ventured to follow the ungodly loose code of life of the Gentiles, God punished them severely. It is not that God did not care for the Gentiles (Isaiah 56:3-7), but He chose the Jews as a holy chosen people (Exodus 19:5, etc). But, as we read in Isaiah, God always welcomed the Gentiles and even promised them better consideration than His own chosen people if they decided to convert to Judaism and do His will. Verse 4 implies that God did not allow the converted Gentile to continue living in his pagan ways while He forbid the Israelites not to live the pagan life. The converted Gentile and the Jew has always been obligated to keep doing His will as revealed to Moses (Isaiah 56:3-7) even today ( Parable of the Rich man and Lazarus in Luke 16:19-31).