Being a Jew is not just about ethnicity. Remember how the Jewish nation of Israel started.
It started with Abraham, the father of faith.
"Even so Abraham believed God, and it was reckoned to him as righteousness. Therefore, be sure that it is those who are of faith who are sons of Abraham. The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, 'All the nations will be blessed in you.' So then those who are of faith are blessed with Abraham, the believer." (Galatians 3:6-9)
God blessed Abraham because of his faith. Abraham believed in an early version of the gospel, because he had faith in God's promise that God would send blessing to all the nations through Abraham's son. It is this Son of Abraham whom we also believe in. That is, Jesus Christ, Son of Abraham, Son of David, who came to die for our sins and save those who believe.
So it is because of this faith, we are all saved, along with father Abraham who is the father of faith.
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Abraham became the father of the Jewish nation of Israel.
A Gentile can become a Jew by faith. To God, becoming a Jew means believing in His promises.
The Bible says:
"And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise." (Galatians 3:29)
"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 which is of the heart, by the Spirit, not by the letter; and his praise is not from men, but from God." (Romans 2:28-29)
God cares about the heart rather than outward appearances or strict ethnicity.
So those whose hearts love God are blessed by Him, whether they are Jew or Gentile. And a Gentile person can become a Jew inwardly by believing in Jesus the Messiah and following Him.
In Esther's time, a Gentile person became a Jew by believing in God's promises of a coming Savior and following His commandments.
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We do not believe in replacement theology, in which the church replaces Israel or that Gentiles can replace Jews.
See below on how replacement theology is wrong:
https://www.gotquestions.org/replacement-theology.html
Rather, we believe that God has good promises and plans for both Jews and Gentiles. God's plan for Israel is ongoing. God is waiting for the Jewish people to accept His Son as Messiah, just as God is waiting for non-believers from other nations to come to believe in Him.
In God, we are all equal before Him.
“For there is no partiality with God.” (Romans 2:11)
“For there is no distinction between Jew and Greek [i.e. Gentile]; for the same Lord is Lord of all, abounding in riches for all who call on Him…” (Romans 10:12)
And God's plan is that Jews and Gentiles become one in unity and love:
“There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus” (Galatians 3:28).
God's plan is that there be no division among us. His plan for our future is that we all unite in love.
Just as a husband and wife is considered one body, so God's desire for all of us believers in Christ is to be one in unity.
Love accomplishes this.