16 From now on, therefore, we regard no one according to the flesh. Even though we once regarded Christ according to the flesh, we regard him thus no longer.17 Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.
Once, according to the flesh and to fulfill His Father’s will, Jesus was a Jew. He has fulfilled that prophecy, died, and rose from the dead returning to His true and eternal self, the only begotten Son of God. Anyone who is in Christ has also died, and now is alive for true and a new thing. Not the old thing.
It’s commonly taught that we Christians are the “New Jews” but that is only true in the sense that Jew is a metaphor for God’s People. In fact it was never the case that genetics was the basis of who was a member of God’s People. Abraham, a Chaldean, was chosen by God, adopted, and was sealed by circumcision. The promise that was given to him was given under the law by which I mean the order of, and according to, the material world of which the law God gave Moses corresponds. In his letter to the Galatians, St. Paul says it this way:
23 Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. 24 So then, the law was our guardian until Christ came, in order that we might be justified by faith.
That law included all sorts of rules about how the people of Israel were to interact with–and prohibit–foreigners. This post is already going to be very long and it would bloat to tens of thousands of words if I included them all, but here is an example which takes place just before the Israelites flee Egypt, and before the law is given:
43 And the Lord said to Moses and Aaron, “This is the statute of the Passover: no foreigner shall eat of it, 44 but every slave that is bought for money may eat of it after you have circumcised him. 45 No foreigner or hired worker may eat of it. 46 It shall be eaten in one house; you shall not take any of the flesh outside the house, and you shall not break any of its bones. 47 All the congregation of Israel shall keep it. 48 If a stranger shall sojourn with you and would keep the Passover to the Lord, let all his males be circumcised. Then he may come near and keep it; he shall be as a native of the land. But no uncircumcised person shall eat of it. 49 There shall be one law for the native and for the stranger who sojourns among you.”
It looks like a set of rules with principled exceptions, but in reality there are no exceptions to the rule that the Passover is to be kept by Israelites and none of it given to foreigners or hired workers. Period. What looks like exceptions are actually instructions of how those slaves and foreigners, by faith, become Israelites; because one who is circumcised in faithful obedience to God is as Abraham himself. There shall be one law for the native and the stranger who sojourns among you. That law is faith in God which produces obedience. One law for one people.
In his letter to the Romans, St. Paul explains this (of which this is only a short bit)
10 How then was [righteousness] counted to him? Was it before or after he had been circumcised? It was not after, but before he was circumcised. 11 He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well, 12 and to make him the father of the circumcised who are not merely circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised.
Whoever shares one father is from the same nation; the same ethnicity. Under Abraham that ethnicity is one of faith, but under Christ, who is the fulfillment of that faith, we truly share in blood. St. Paul explains it magnificently in his letter to the Ephesians:
11 Therefore remember that at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands— 12 remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. 13 But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. 14 For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility 15 by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, 16 and might reconcile us both to God in one body through the cross, thereby killing the hostility. 17 And he came and preached peace to you who were far off and peace to those who were near. 18 For through him we both have access in one Spirit to the Father. 19 So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, 20 built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, 21 in whom the whole structure, being joined together, grows into a holy temple in the Lord. 22 In him you also are being built together into a dwelling place for God by the Spirit.
Made us both Jew and Gentiles one. Fellow citizens. Members of the household of God. One temple. One people. He writes the same to the Galatians because of their struggles with the anti-Christian Jews who try to ritually ensnare those who formerly were Gentiles in the flesh. The end of his thought which I quoted above is powerful and direct:
23 Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. 24 So then, the law was our guardian until Christ came, in order that we might be justified by faith. 25 But now that faith has come, we are no longer under a guardian, 26 for in Christ Jesus you are all sons of God, through faith. 27 For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. 29 And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
It wasn’t that Gentiles became Jews, but that both are done away with in Christ and we, and our blood, are now something new and better. There are still Jews and Gentiles in the world, and for we who are in Christ mixing with them, marrying and having sex with them, it is miscegenation and a sin.
There are sinful compulsions which are particularly rampant among Jews, and the various Gentile nations are each bedeviled by sins to which they are respectively given. St. Paul, while counseling Titus, speaks of both Jews and Cretans in a race-realist way that would cause wailing and gnashing of teeth in most churches:
10 For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. 11 They must be silenced, since they are upsetting whole families by teaching for shameful gain what they ought not to teach. 12 One of the Cretans, a prophet of their own, said, “Cretans are always liars, evil beasts, lazy gluttons.” 13 This testimony is true. Therefore rebuke them sharply, that they may be sound in the faith, 14 not devoting themselves to Jewish myths and the commands of people who turn away from the truth. 15 To the pure, all things are pure, but to the defiled and unbelieving, nothing is pure; but both their minds and their consciences are defiled. 16 They profess to know God, but they deny him by their works. They are detestable, disobedient, unfit for any good work.
Jews are deceitful, gossiping swindlers; Cretans are lying, lazy, evil, gluttonous beasts; both are good for nothing. They are not in Christ. Those who were formerly Jews and formerly Cretans are in Christ, and they are something new in one new nation which is of Christ’s blood; as are we.
 See: Wineskins
 So if someone says “Those cretans jewed me out of my money.”, we should assume that person is somewhat biblically literate.