Hi Rabbi; Recently I had a discussion with a christian friend of mine over whether or not Abraham was the first Jew. I said he was because God’s covenant was with him . My friend contends that the three religions began with Abraham and therefore he can’t be the first Jew. How I can I refute his arguement.
Avraham is considered to be the first Jew, though Ishmael is his biological son and Eisav (Esau) his biological grandson, Avraham gave his legacy to Yitzchak (Isaac) and made him his successor. Same with Yaakov (Jacob) and Eisav, Yaakov was chosen and Eisav rejected. Hashem tells Avraham explicitly that only Yitzchak is to be called his seed as stated in Genesis 21; 12. Same is with Yaakov ibid. 28; 4 and more. All of our three forefathers were not Jews by descent but by choice, only from Yaakov and onwards were all children Jews. See more in "The Kuzari" by Rabbi Yehuda Halevi 1; 47.