The descendant of Abraham, Isaac & Jacob... are the Jews. Isaac was born from a legitimate wife Sarah according to promise.
Pentateuch (5 book of Moses, including the verse you quoted form Genesis 15:18). God's people keep Sabbath even till this day. The Jews.
The Arabs are the descendant Ismael from illegitimate maidservant/slave Hagar, who was casted out by Abraham. Galatians 4:30, "Nevertheless what does the Scripture say? “Cast out the bondwoman and her son, for the son of the bondwoman shall not be heir with the son of the freewoman.” Today they are known as the Arab (non-sabbath keeper).
