"Father Abraham had many sons, many sons had father Abraham. I am one of them and so are you...," so the old children's song goes. Although it's a silly song that requires the raising of arms, legs, and the nodding of heads, the song's theology is unquestionably biblical. In Romans 4:1–16, the Apostle Paul explains that Abraham is "the father of us all."
As new covenant believers, we have been justified by grace alone through faith alone because of the finished work of Christ alone. As an old covenant believer, Abraham, the father of Israel, was justified by grace alone through faith alone because of the future work of Christ alone. Abraham stood on the promised salvation of God in the Messiah who was to come just as we stand on the promised salvation of God in the Messiah who has come. True believers in the Old Testament were saved in the same way that true believers are saved in the New Testament— by faith, and by faith alone. Abraham believed the Lord, and He counted it to him as righteousness (Gen. 15:6). The Lord declared Abraham righteous because he believed, just as the Lord declares us righteous because we believe. Thus, adoption into God's family and eternal covenant community is achieved not through having the right family name, ethnicity, land of birth, or residence. Neither are men and women the children of Abraham and, therefore, the children of Israel because of Sabbath-keeping or unconditional support of all of Israel's practices and policies. True Israel is faithful Israel, and only faithful Israel inherits God's promises. And faithful Israelites are those circumcised in their hearts, those who have trusted in the Messiah. This is the way God has always fulfilled His purposes in saving His people (Rom. 2:28–29).
True Israel is faithful Israel because they have faith in the only faithful Israelite who has ever lived—Jesus the Messiah. Only Jesus completely fulfilled all of the Father's righteous laws for Israel. As the only faithful Israelite, Jesus is an Israelite according to the flesh, and He enjoyed all the benefits that come from being born into the nation that possessed the oracles of God. As the faithful Israelite Jesus is the true Israel because He is the true Son of God (Matt. 2:13–14).
By faith in Jesus, the true Israelite, all people can be reckoned as true Israelites. All who are united by faith alone to Jesus the Christ are the true Israel of God (Gal. 6:16). As such, those of ethnic Israel who trust Jesus as their Messiah are closer to me than they are to their Jewish families who don't believe in Jesus. Likewise, I am closer to Jewish believers in Christ than I am to my unbelieving Gentile family. For our Father has graciously made us His children having freed us from bondage in order to live coram Deo, following Jesus, the true Israel of God.