Could you explain to me about the chosen nation either in Old Testament and New Testament?
The Jews are known in the Old Testament as a "chosen people." They were chosen by God because of the faith of Abraham. The promise that the family of Abraham would be chosen is found in Genesis 12:1-7, Isaiah 15:2-5, and Isaiah 17:1-8. Many passages in the Old Testament can be used to show that the Jews are God’s chosen people. Examples include Deuteronomy 9:4-6, Genesis 18:16-19, Deuteronomy 7:6 and many more.
In the New Testament the idea of being a chosen people is found as well. Through the work of Jesus, the chosen people of God became, not the Jews, but those who are baptized into Christ. 1 Peter 2:9 says that all those who are in Christ are a chosen people of God.