Likewise, Jews who become Christian/Catholic do not cease to be Jews.
Who gets to decide that, would you say?
The Apostles taught that the Jew and Gentile are made one new creation with another
in Yeshua Ha’Mashiach.
But until the resurrection we still are in our natural, “first” bodies, though we have been born again in the Spirit of the God of Israel.
**1 Corinthians 15:42-49
So will it be with the resurrection of the dead. The body that is sown is perishable, it is raised imperishable; it is sown in dishonor, it is raised in glory; it is sown in weakness, it is raised in power; it is sown a natural body, it is raised a spiritual body.
If there is a natural body, there is also a spiritual body. So it is written: “The first man Adam became a living being”; the last Adam, a life-giving spirit. The spiritual did not come first, but the natural, and after that the spiritual. The first man was of the dust of the earth, the second man from heaven. As was the earthly man, so are those who are of the earth; and as is the man from heaven, so also are those who are of heaven. And just as we have borne the likeness of the earthly man, so shall we bear the likeness of the man from heaven. **
That being said, because we still our in our natural bodies, of course then a Jew is still a Jew and a Gentile is still ethnically whatever country they are from. Though the New Covenant both partake in through Yeshua is explictably Israeli, for Yeshua is the one and only Son of the God of Israel, and the New Covenant is announced to Israel(Jeremiah31:31-34).
So for a Jew to become one new creation with the Gentile in Yeshua does not severe him or her from their Jewish heritage. The scriptural appointed times, among other things, are done in the fulfillment and celebration of Yeshua. Christians who are Jews celebrate the fact that the God of Israel has revealed how He reconciled Jew and Gentile, by His Son, to be one new creation in Him.
The Apostles taught that the Jews must not impose any “works” to add to the Grace given to the Church. The Church from Jerusalem gave the Gentile Christians a few instructions when they were faced the problem of some imposing the Law of Moses on them-
Acts 15:28-29
It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements: You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things.
To conclude and to answer your question, the Apostles taught our Lord and God decides this-
**1 Corinthians 7:17-20
Nevertheless, each one should retain the place in life that the Lord assigned to him and to which God has called him. This is the rule I lay down in all the churches. Was a man already circumcised when he was called? He should not become uncircumcised. Was a man uncircumcised when he was called? He should not be circumcised. Circumcision is nothing and uncircumcision is nothing. Keeping God’s commands is what counts. Each one should remain in the situation which he was in when God called him. **