Let’s simplify your argument:
A. Jews are still the chosen people
B. Jews follow the mosaic Law
Therefore:
C. following the mosaic law makes them the chosen people.
And:
D. Christians are the chosen people
E. Christians do not follow the mosaic law
Therefore:
F. they are the chosen people regardless of the mosiac law.
So which is it?
Does following the mosaic law make a person “chosen” or not? If God is perfectly absolute and perfectly just, why would He have two sets of rules? The old law is either necessary or it is not. No in-between.
We are judged by faith and not by works of the law.
Point C does not follow from Points A and B.
Do you not understand that it is the other way around? That is, Jews are not chosen because they follow the Mosaic law. Rather, Jews understand themselves to be held to the higher standard of the Mosaic law
because they have been chosen. If you tell a Jew you are going to convert to Judaism, in fact, it is not unusual to have them express surprise that you’d want to take than on. So what makes you one of the Chosen People? That you are descended from Abraham, and from the people who have freely joined the families of his descendents, as Ruth and Rahab of Jericho did. That is all. You might freely join the Chosen People by converting, but other than that there is not a bit of belief or behavior implied. A Jew born a Jew is a Jew, period.
This is why Jews do not attempt to gain converts or try to make the entire world keep kosher. They do not teach that those who are not “chosen” aren’t going to gain eternal life. To the contrary: righteous Gentiles are not held to the same standards as Jews, but to something akin to what we would call “natural law.” The Book of Life is not believed to hold only the names of Jews.
Furthermore, Scriptures are full of evidence that God may justly choose to have different rules for different people, particularly with regards to ritual requirements, since He makes one vessel for this use and another for that one. Is it “fair” that those who worked one hour in the vineyard receive the same wage as those who worked the full day in the sun? God is compassionate and generous, as He sees fit. There is no injustice in that!
Do we not treat sacred vessels different from the profane? Does this mean the profane vessels have no use? No, it means only that they are given a dignity in accord with their use. We have the same thing within Christianity, do we not, with consecrated religious? They make vows to which the rest of us are not bound, having been set aside, as it were, for sacred use. This does not make them automatically more united to God, for that is a function of obedience to the use to which each has been called.
God will judge each one according to how each one’s heart is read by its Maker. You could look at that as there having been as many “different sets of rules” as there are souls, if you were so inclined. In reality, the rule is this: to love the Lord your God with all your heart, with all your strength, and will all your mind, and to love your neighbor as yourself. It is not just Jesus who taught this. The Rabbi Hillel taught it, as well. What that means in any particular case is the work of each soul to pursue with every diligence.
One who sees that Jesus is Lord and yet will not follow, or who could see except that he refuses to open his mind to the possibility, this is a person who is withholding himself from God. This is a serious refusal. But only God can read a soul to decide to what extent this refusal is willful. God only will judge. That is not our job. Our job is to be a light by which others may see. The rest is in God’s hands, for God alone is just.