There are three “Abrahamic” faiths. Judaism, Christianity, Islam alone among world religions trace the beginnings of their story back to a single patriarch, all of whom claim to be his true heirs, interpreted variously of course. But herein lies the fundamental difference between Christianity on he one hand and Judaism and Islam on the other. For Christianity it is precisely not legitimate ethnic or national descent from Abraham that places one within the people of God. This is a distinctly biblical point. God is able to raise up true children of Abraham from stones (Luke 3:8), it is the Spirit that gives life, the flesh is of no value (John 6:53). The incorporation of persons into the people of God according to the Christian faith, be they Jew or Gentile is always a distinctly unnatural event.
This is clear in the Pauline corpus with regard to the Gentiles (see especially Romans 11 cf. Eph 2:12-13). However the Johannine corpus goes even further in arguing that being reborn into the true community of God’s people is a miraculous novum, not just for Gentiles, but for the Jews as well. “He came to what was his own, and his own people did not accept him. But to all who received him, who believed in his name, he gave power to become children of God, who were born, not of blood or of the will of the flesh or of the will of man, but of God” (John 1:11-13). The people of God, in John’s gospel in particular are constituted not by any sort of legitimate, natural, historical succession. Rather the people of God are constituted by the miracle of being reborn by the Spirit into a radically new community that interrupts and scandalizes all “natural” communities.
Thus, from the perspective of inter-religious dialogue, one key element that distinguishes Christianity from the other two Abrahamic faiths is Christianity’s explicit denial of what the other two vehemently claim, namely that “legitimate” descent from Abraham substantiates the claims of their faith. Christians make no claim to be legitimate children of Abraham, rather they claim that their status as God’s people is derived from nothing inherent within themselves either ethnically or politically. We are the people of God solely and only because of the radical miracle brought about in Jesus Christ which shatters and scandalizes any “natural” claim to be God’s people through historical natural succession.