Now we, brethren, as Isaac was, are the children of promise. Paul here was writing to gentile and jewish believers, all who belonged to Christ, Abraham's seed Gal 3:29 29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise [As Isaac was]. We see also see the phrase identification " The Children of Promise" in Paul's writing in regard to the Two Israel's in Rom 9:6-8 6Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: 7Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. 8That 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. Are Gentiles Children of Promise ? Yes, by all means, else would Paul's comments in Gal 4:29 be insignificant ! How are Gentiles Children of Promise and Israel and not just ethnic jews ? Lets look at some scripture as it relates to Abraham and promises God made to Him. Gen 17:1-5 1And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect. 2And I will make my covenant between me and thee, and will multiply thee exceedingly. 3And Abram fell on his face: and God talked with him, saying, 4As for me, behold, my covenant is with thee, and thou shalt be a father of many nations. 5Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. First of All, God identified Himself to Abram as El Shadday, The LORD Almighty, emphasizing His Power, as such God promises Abraham, that He would multiply him greatly [ see Gen 13:16 also] This is an unconditional Promise to Abraham to multiply His Offspring, as He is the Almighty. The word multiply is the hebrew word rabah and means: be or become great, be or become many, be or become much, be or become numerous to multiply, increase God had Promised Abraham Children, an offspring, descendants, this was according to God's Power and Promise much as with His Promise to Abraham in regards to Isaac. Now many have taken this promise in Gen 17:2 and applied it to Abraham's Natural Offspring, His Children of the Flesh, The Children of Israel according to the Flesh, but that is not the case, that is a false assumption, God is here promising Abraham a Spiritual Offspring, an Offspring that is of Faith, Just as He was made to be. This Offspring was given to Abraham, not as the Father of the Ethnic jews, but as the Father Spiritually of All Nations, this is seen in verses 4-5 of Gen 17: 4As for me, behold, my covenant is with thee, and thou shalt be a father of many nations. 5Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. All People, whether jew or gentile who come to Faith or believe are the Children of Promise. Their coming to Faith is by the Power of and promise of El Shadday, The Almighty ! The writer of Hebrews refers to this promise here Heb 6:12- 14 12That ye be not slothful, but followers of them who through faith and patience inherit the promises. 13For when God made promise to Abraham, because he could swear by no greater, he sware by himself, 14Saying, Surely blessing I will bless thee, and multiplying I will multiply thee. The writer is Identifying those in vs 12, them who through Faith and Patience inherit the promises, as of that seed that God sware to Give to Abraham when He said to Him vs 14 Surely blessing I will bless thee, and multiplying I will multiply thee. These are the Children that God promised to Abraham that He had made him a Father of many Nations, and when He first called his name Abraham Gen 17:5 5Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. So Abraham, as God promised, is the Father of all who believe: Rom 4:11,16 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also. Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, See, both jew and gentile believers are of one promised seed of Abraham. So, all True Believers, whether jew or gentile are the Children of Promise [Israel of God], born out of God's Promise to Abraham: Gen 17:2 2And I will make my covenant between me and thee, and will multiply thee exceedingly.