Do Gentiles become Jews when they believe?

 As a believer (Gentile) you become part of the body of Christ, not Israel. You are a spiritual Gentile, not a Jew.  Israel is a nation consisting of Jews, Hebrews are an ethnicity who are from the linage of Abraham Isaac and JACOB (the twelve tribes). A gentile (both believer and non believer) is from Abraham not Isaac and Jacob.

Being Abraham’s seed is not make one Jewish, this is where people have been taught wrong, its Abraham Isaac and Jacob, (from whom came the 12 tribes) otherwise the Arabs are Jewish too. selah.

John 18:35 Pilate answered, "Am I a Jew? Your own nation and the chief priests have delivered You to me.”

Rom 1:16 “For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek.”

It is written "that the Christ would suffer, that He would be the first to rise from the dead, and would proclaim light to the Jewish people and to the Gentiles." (Acts 26:23). Two groups of people. Later it was Saul who was specifically called to this other group while the original apostles worked among their people the Jews. Paul was also going to the Jews first as his priority but they rejected the message  from him and she said in Acts 28:28 "Therefore let it be known to you that the salvation of God has been sent to the Gentiles, and they will hear it!"

Rom. 11:12-13 “Now if their fall is riches for the world, and their failure riches for the Gentiles, how much more their fullness! For I speak to you Gentiles; inasmuch as I am an apostle to the Gentiles, I magnify my ministry. ” He spoke of himself as “a teacher of the Gentiles in faith and truth”(1 Tim. 2:7)

Paul made the distinction of Gentiles and Jews before their salvation and continue this distinction afterwards, in Rom.2:9,10.

Rom. 10:12-13 “For there is no distinction between Jew and Greek, for the same Lord over all is rich to all who call upon Him. For "whoever calls on the name of the Lord shall be saved."

 He is not saying Jews and Gentile are no longer valid or erased, but that both call upon the Lord for salvation.

Under the new covenant”

Rom. 2:28-29 “For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; but he is a Jew who is one inwardly; and circumcision is that of the heart, in the Spirit, not in the letter; whose praise is not from men but from God.”

In other words one must have faith to be a Jew, not just in their lineage. It was also a matter of the heart.

Speaking to the gentiles, Gal 3:29 “And if you are Christ's, then you are Abraham's seed, and heirs according to the promise.” He refers to one being of the seed of Abraham as one exercising faith.

Paul had to rebuke Peter:

Gal 2:14 But when I saw that they were not straightforward about the truth of the gospel, I said to Peter before them all, "If you, being a Jew, live in the manner of Gentiles and not as the Jews, why do you compel Gentiles to live as Jews?

 Clearly there were distinctions, yet he also said of those who were in the body of Christ to not see each other as they did before.

Col. 3:10-11 “and have put on the new man who is renewed in knowledge according to the image of Him who created him, where there is neither Greek nor Jew, circumcised nor uncircumcised, barbarian, Scythian, slave nor free, but Christ is all and in all.”

Gal 3:28 “There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus.”

What he is saying is that we are all children of God, the body of Christ and that any differences should not be used to divide (2 Cor.12:13). Jew and Greek is a more specific substitute for the distinction of Jew and Gentile. There were still slaves and masters existing at the time just as there were still men and women. But this was to be overlooked in defining a person who is now in christ.

In the same way Matt 19:6 "So then, they are no longer two but one flesh. “ Does not mean one literal person, but spiritually.

Jesus says Rev 2:9 "and I know the blasphemy of those who say they are Jews and are not, but are a synagogue of Satan.

Clearly there were those who claimed to be Jews but were not believers and were against the Messiah (as in Acts 14:2).

The Gentiles as said to be called by his name (Acts 15) Acts 21:25 “Gentiles who believe”

Paul never said Gentiles were Jews after they believe

Rom. 15:10-11 And again he says: "Rejoice, O Gentiles, with His people!"And again: "Praise the Lord, all you Gentiles! Laud Him, all you peoples!"

Gal 3:14 that the blessing of Abraham might come upon the Gentiles in Christ Jesus, that we might receive the promise of the Spirit through faith.”

Eph 3:6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel”

Acts 17:17 Therefore he reasoned in the synagogue with the Jews and with the Gentile worshipers, and in the marketplace daily with those who happened to be there.”

Just as Gentiles were not called Jews when they worshiped God in the synagogue, they are not called Jews when they believed in Messiah Jesus. They are fellow heirs, they become believers, spiritual gentiles. this distinction continued through the Scripture.