Romans 9:8

That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed.

With what deliberate caution Paul approached the dreadful announcement he was obligated to deliver to his beloved kinsmen! He first laid the logical support of what he had to say by citations from the Old Testament scriptures, and then built up the premises upon which he would rest his conclusion. This verse spells out the deduction to be made from the history of Abraham's sons, only one of which, namely, Isaac, was his true seed, all the others being rejected. Just so it is today, Paul was saying, not merely the fleshly children of Abraham are his seed, but the children of the promise, this reference to the promise pointing to Genesis 12:3, where not Jews only, but "all the families of the earth" were to be blessed.

Children of promise ... has in view the fact that Isaac was not born in the due course of nature, but in respect of God's promise, which was providentially fulfilled when both Abraham and Sarah were long past the age of child production. This fact regarding Isaac is typical of Christians who, in another sense, are children of Abraham, by promise, as stated thus by Paul:

Now we brethren, as Isaac was, are children of promise .... And if ye are Christ's, then are ye Abraham's seed, heirs according to the promise (Galatians 4:28; 3:29).

Paul's argument from this, of course, was that, just as Ishmael did not inherit, though a literal son, the Jews of Paul's day might not inherit, unless their claim was founded on something else, other than fleshly descent from Abraham. Only those who received and accepted God's promise to Abraham of the Seed which is Christ, and honored and obeyed him, now that he had appeared upon the earth only those persons (the Christians) were the true children of Abraham and heirs according to the promise.