Whoever believe that Jesus is The Son of God is the chosen one. Because Jesus is The Chosen One, and whoever believe in Him is counted among His flock : They are the chosen ones.
The case was different when God had not incarnate in a man: He always identify Himself as “the God of Abraham Isaac and Jacob”. After Jesus come, God introduce Himself in The Name of His Only Begotten Son (Jesus). And those who recognize God’s Spirit in The Name of Jesus, The Messiah, The Son of The Living God, are the chosen people. Therefore, we always identify God’s Spirit in the name of His Only Begotten Son.
About the Jews :
Based on letters, Jews are the chosen ones, although up to now they are still searching for the messiah. It is also written that “God will never forget Israel” : Jews are the literal Israel, Abraham’s descendant, the firstfruit, according to flesh and blood.
However, we’ve come to know that the true “firstfruit” is Jesus Christ, the only begotten Son of God, and therefore His follower should be the true Israel.
The basic principle of “The chosen ones” is always come back to “according to Faith” and never “according flesh and blood”. This teaching is made obvious when God asserted to Abraham that the chosen one was “Isaac” and not “Ismael” (Genesis). Later, Paul explained that Isaac is the son of “the free woman” and Ismael is the son of “a slave”, and that the chosen ones have to be Abraham’s descendant from the free woman, that is to say “according to Faith”, and not the son of a slave ( “the son according to flesh and blood”).
God bless you.