Why does God call Abraham the father of the Jewish people if he wasn't Jewish?
God calls Abraham the father of the Jewish people by His sovereign decree, not because Abraham was ethnically Jewish. Abraham came from Syrian parents, but God separated him from his family in Ur and declared him father of the Jewish race. The first actual Jew, technically, was Isaac.
Les explains that the Gentile stream of humanity comes from Adam, through the three sons of Noah, and through the nations scattered at the tower of Babel. Out of that stream God pulled the little nation of Israel by separating Abraham. Abraham was not a Jew; he was born of Syrian parents. Isaac went to Haran and married Rebekah, a Syrian, and Jacob later got his wife from Laban, who was still a Syrian. But by God's decree and sovereign work, He declares Abram the father of the Jewish race. The Jew begins when God separated Abram out of his family in Ur of the Chaldees, and by his wife, a half sister, he had Isaac. So technically the first real Jew would be Isaac, yet God refers to Abraham as the father of the Jewish people even though he came from one of the other nationalities.