The author is a Presbyterian minister who presumably believes that the nation of Israel is now the spiritual decendants of Abraham.
Romans 9:6 Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: 7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. 8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
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).