For your information, we Presbyterians do not believe that the nation of Israel are the spiritual decendants of Abraham. Abraham's spiritual decendants are based on faith not on genetic lineage (read Romans Ch 9, and Hebrews Ch 11), otherwise they would only be physical decendants. The church (Christians) are reckoned to be the spiritual decendants of Abraham (Romans 9).
I guess I did not make myself clear: Israel is the spiritual decendents of Abraham not the genetic lineage.