Abraham is not the forefather of Rabbinic Jews. He is the Father of all Christians, as he believed in the True God.

"Holy Abraham, our father according to the flesh through Isaac, the son of promise whom you gave up to death in order to obey the voice of God and who, being saved by the Our Lord Christ himself in the form of an angel, became the father of the Christian race, we cry out to you and pray to you: intercede with your God and ours to be merciful to us, to gently correct our falls, to lift us up strongly from despair, to embrace us in His Fatherly Arms, and to give us the grace of faith.

Becoming children of God through faith in Jesus Christ, we praise the Great One among the Saints, the Son of God, and we pray to you, Holy Abraham, who are the father of all who believe in the True God."

https://orthodoxreflections.com/akathist-to-saint-and-righteous-patriarch-abraham-from-the-old-testament/