“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.”
Paul is arguing that there is more to Jewishness than mere descent from Abraham; that the chosen people were not simply the sum of all the physical descendants of Abraham.  That is, spiritual kinship, not ethnic origin, determined who was a true Israelite.

          “They which are the children of the flesh”
           Literally: “That is, not the children of flesh.”–The natural descendants.

         Not all the children who descend from Abraham's loins, nor all those who were circumcised as he was are therefore people of God; but those who are made children by the good pleasure and promise of God, as Isaac was, are alone to be accounted for the seed with whom the covenant was established.
Ishmael, born according to natural laws, was not of the chosen race, but rather Isaac, the child of promise, born contrary to natural law, became the one chosen by God, it stands to follow that the children of
God are not the children of the flesh.  That is, not simply the mere fleshly descendants of Abraham, are the children of the promise;   rather, the children of the promise  are those who are of the seed according to the conditions of the promise. This argument is a reply to those who based all upon their fleshly relation to Abraham: “We have Abraham to our Father” (Matt. 3:9). In order to show this more fully Paul recalls the incidents recorded in Gen. 18:10-14.

“these are not the children of God”
Literally: “These {are} not children of God.”–Meaning
Abraham's children by mere natural  descent. These are not of necessity the adopted children of
God ; or are not such by virtue of merely their natural descent.

                 CHILDREN OF GOD(Grk–tekna tou Theou)–Those in a special sense, as being     heirs of the promises made to Abraham.

This was in opposition to one of the most settled and deeply-cherished opinions of the Jews.  They supposed that the mere fact of being a Jew entitled a man to the blessings of the covenant, and to be regarded as a child of God.  But Paul shows them that it was not by their natural descent that these spiritual privileges were granted; that they were not conferred on men simply from the fact that they were Jews; and that consequently those who were not Jews might become interested in those spiritual blessings.

         “the children of the promise”
         Literally:  “But the children of the promise.” The descendants of
Abraham on whom the promised blessings would be be-stowed.

         CHILDREN OF PROMISE:  (Grk.–tekna tēs epangelias)—Only the children of    the promise are, in the full sense, “children of God” (Grk.--tekna tou theou).  The descendants of Isaac, who was born in a supernatural way, according to God's promise, which is mentioned in the following verse.  They to whom the promise is made, that is, true believers,   are the children of God.

Paul is not speaking of Christians here, but simply showing that the privileges of the Jews were not due to their physical descent from Abraham (cf. Luke 3:8).  The children of Isaac are counted for the seed; the seed that inherits the promises made to Abraham .

         “are counted as the seed”
         Literally: “{Are}counted for a seed.”– They to whom the promise is made, that is,  believers,  are the children of God.

                        COUNTED:  (Grk.–logizetai)-Literally:  {are} regarded, or reckoned.

“For this is the word of promise, “At this time will I come, and Sara shall have a son.’”

         “For this is the word of promise”
         Literally: “For the word of promise”--Paul combines Gen. 18:10, 14 from the
LXX  (Septuagint). This promise was made when Sarah was far beyond the natural age of bearing children, and when
Abraham was an old man. So the chosen seed are children of promise.

                 WORD OF PROMISE:  (Grk.–epangelias ho logos houtos)– Literally, "this word  is one of promise."

Paul’s purpose in introducing this seems to show to whom the promise pertained; and by specifying this he shows that it had no reference to Ishmael, but to Isaac. By the power of which Isaac was conceived, and not by the power of nature.

“at this time I will come”
“According to this time I will come”–At the time which I now appoint.  (See Gen. 8:10, 14). This probably means “at the exact time promised”  I will fulfil the prediction at the very time (comp. II Kings 4:16).

That is, this is evidently implied in the promise in Gen.18:10: “At this time I will come, saith God,” and exert My Divine power, “and Sarah, though fourscore and ten years old, shall have a son;” which shows that it is the sovereign will and act of God alone, which singles out and constitutes the peculiar seed that was to inherit the promise made to Abraham.

                 AT THIS TIME:  (Grk.–kata ton kairon touton)–Literally: “According to this season.”

“And not only this; but when Rebecca also had conceived by one, even by our father Issac”

“And not only this”
“And not only {so}.”–Not only is the principle of making a distinction among the natural descendants of Abraham thus settled by the promise, but it this further seen in the birth of the two sons of Isaac. 

          NOT ONLY THIS:  (Grk.–ou monon)–“Not only {so}”–The thought here is, not only have we an example of the election of a son of Abraham by one woman, and a rejection of his son by another, but also of the election and rejection of the children of the same woman.

“when Rebecca also had conceived by one”
Literally:  “Rebekka having in womb from one”–Paul shows that it is not according to any natural principles that the selection was made; that God made a distinction between Jacob and Esau.

         God chose according to His good pleasure, choosing the younger to be the object of His favor, and rejecting the elder, who, according to the custom of the times, and the law of primogenitor, was supposed to be entitled to peculiar honor and rights.  Paul mentions this to show that neither were their ancestors accepted through any merit of their own.
         The first argument is that the true seed are children of the promise, a spiritual seed rather than of the flesh. The second argument, now begun, is that God has the right to reject what nation He so wills, including the Jews, and to choose other races if He wills.  This is shown by facts from history. He did exercise the right of choice when He chose Jacob as the father of the chosen nation, instead of Esau. The facts are recited to show this.

“our father Issac”–It might be thought that there was a natural reason for preferring the child of Sarah, as being Abraham's true and first wife, both to the child of Hagar, Sarah's maid, and to the children of Keturah, his second wife.

There could be no such reason in the case of Rebecca, Isaac's only wife; for the choice of her son Jacob was the choice of one of two sons by the same mother and of the younger in preference to the elder, and before either of them was born, and consequently before either had done good or evil to be a ground of preference: and all to show that the sole ground of distinction lay in the unconditional choice of God —“not of works, but of Him that calleth.”

“For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of Him that calleth.”

For the children being not yet born”
Literally:  “For {the children} not yet having been generated.”–As the word “children” is not in the original Greek text, the word nations would be more proper; for it is of nations that Paul speaks, as the following verses show, as well as the history to which he refers.

         It was not by any works of theirs, nor was it because they had formed a character and manifested qualities which made this distinction proper. It had its foundation in the purpose or plan of God.  In order to prove that this was done according to God’s own pleasure, Paul shows that the distinction was made before they were born; before they had formed any character; and, consequently, in such a way that it could not be pretended that it was in consequence of any works which they had performed.
         Paul had shown that the principle of making a distinction among the posterity of Abraham was recognized in the original promise, thus proving that all the descendants of Abraham were not of course to be saved; and he now proceeds to show that the principle was recognized in the case of his posterity in the family of Isaac.

         “neither having done any good or evil”
         Literally: “Nor having done any good or evil.”–That is, when the declaration (v. 12) was made to Rebecca..

This is a very important passage in regard to the question about the purposes of God. They had done nothing good or bad; and when that is the case, there can be, properly speaking, no moral character, for “a character is not formed when the person has not acquired stable and distinctive qualities.” Neither having done any good to merit the distinction of being made the peculiar people of God; nor evil, to deserve to be left out of this covenant, and the distinguishing national blessings which it conferred; that the purpose of God according to election might stand-that such distinctions might appear to depend on nothing but God’s free choice, not of works, or any desert in the people or nations thus chosen; but of the mere purpose of Him Who calls any people He pleases, to make them the depositories of His especial blessings, and thus to distinguish them from all others.

“that the purpose of God according to election might stand”
Literally:  “That the purpose of God according to election might stand.”–That it might stand forth that
God made the choice of His own will; meaning that of His own volition
God chose Jacob, yet unborn, to become the head of the chosen race, rather than Esau.

God dispensed His favors according to His sovereign will and pleasure. Those favors were not conferred in consequence of the merits of the individuals, but according to a wise plan lying back of the formation of their characters, and before they had done good or evil. The favors were thus conferred according to God’s sovereign choice, or election. Note that this election was not to eternal salvation, but to become the head of a people, as Moses, Samuel, and John the Baptizer were raised up for a great work of God, so was Jacob.

         MIGHT STAND: (Grk.–menēi)–That it might be continued; or remain unchanged. The case shows that God. dispenses His favors as a sovereign. The purpose of God was thus proved to have been formed without respect to the merits of either.

“Not of works”– Not by anything which they had done either to merit His favor or to forfeit it.   So it is in relation to all who shall be saved. God has good reasons for saving those who would be saved.

“but of Him that calleth”
Literally:  But of the {One} calling.”–According to the will and purpose of Him that chooses to dispense those favors in this manner. It is not by the merit of man, but it is by a purpose having its origin with God, and formed and executed according to His good pleasure.

It is also implied here that God’s choosing is formed in such a way as to secure His glory as the primary consideration.  That such distinctions might appear to depend on nothing but God’s free choice; not of works, or any desert in the people or nations thus chosen; but of the mere purpose of Him who calls any people He pleases, to make them the depositories of His especial blessings, and thus to distinguish them from all others.

“It was said unto her, The elder shall serve the younger.”
The source of the selection is God himself. Paul quotes Gen. 25:33 from the  LXX.

         “It was said unto her”
         Literally: “It was said to her.”  By Jehovah God (see Gen. 25:23)–a case of a Theophany. 

“the elder shall serve the younger”
Literally:  “That the greatest shall serve the lesser.”–By the law of primogenitor among the Hebrews, the eldest son, which was Esau, he would have been entitled to peculiar honors and privileges. But it was said that in this case this custom should be reversed, and that he should take the rank of the younger. 

        SHALL SERVE: (Grk.–douleusei)–Shall be subject to; shall not have the authority and priority, but should be inferior. Jacob was to have the priority; the promised land; the promises; and the honor of being regarded as the chosen of God. There was reference here, therefore, to the whole train of temporal and spiritual blessings which were to be connected with the two races of people.

         The passage in Gen. 25:23 shows that this had reference particularly to the posterity of Esau, and not to him as an individual. The sense is, that the descendants of Esau, who were Edomites, should be inferior to, and subject to the descendants of Jacob.
         These words, with those of Malachi, Jacob have I loved, and Esau have I hated, are cited by Paul to prove, according to their typical signification, that the purpose of God, according to election, does and will stand, not of works, but of Him that calls.