Now we, brethren - We who are Christians.
Are the children of the promise - We so far resemble Isaac, that there are great and precious promises made to us. We are not in the condition of Ishmael, to whom no promise was made.

Now we - Who believe in the Lord Jesus, are the children of promise - are the spiritual offspring of the Messiah, the seed of Abraham, in whom the promise stated that all the nations of the earth should be blessed.

Now we, brethren, (g) as Isaac was, are the children of (h) promise.
(g) After the manner of Isaac, who is the first begotten of the heavenly Jerusalem, as Israel is of the slavish synagogue.
(h) That seed to which the promise belongs.

Now we, brethren, as Isaac was,.... The Ethiopic version reads, "you, brethren"; and so one of Stephens's copies. As the two women, Hagar and Sarah, might be, and are allegorized; so likewise their respective offspring. Isaac signified, and was a type and figure of Abraham's spiritual seed, whether Jews or Gentiles, under the Gospel dispensation: and as he was, so they are,
the children of promise; as Isaac was promised unto Abraham, so were this spiritual seed, when it was said unto him, that he should be the father of many nations, as he is the father of us all, even of all them that believe, be they of what nation soever; and as Isaac was born by virtue, and in consequence of a promise made to Abraham of God's free good will and pleasure, and his generation and conception were beyond the strength and course of nature, were the effects of a divine power, and were something supernatural; so this spiritual seed are born again, by virtue, and in consequence of a promise, not only made to Abraham, but to the Lord Jesus Christ, the head of the covenant, that he should see his seed, enjoy a numerous offspring, and which should endure for ever; and also to the church, of whom it is said, that this and that man should be born in her; and particularly in consequence of the promise cited in the foregoing verse, from whence these words are an inference, deduction, or illustration: and these children of the promise, so called from hence, are born again, above and beyond the strength of nature; not through the power and free will of man, but through the abundant mercy and sovereign will of God, by his powerful and efficacious grace, and by the word of promise, the Gospel, as a means. Moreover, to these children, or spiritual seed of Abraham, signified by Isaac, do all the promises belong, as that of God, as a covenant God gave unto them; of Christ, as a Saviour and Redeemer; of the Holy Spirit, as a sanctifier and comforter; and of all good things, of temporal mercies, so far as are for their real good; and of all spiritual blessings, as righteousness, peace, pardon, comfort, all supplies of grace, and eternal life: and these likewise receive these promises; the Holy Spirit, as a spirit of promise, opens and applies them to them, puts them into the hand of faith, and enables them to plead them with God, and to believe the performance of them; so that this character in all respects agrees with them.

The history thus explained is applied. So then, brethren, we are not children of the bond-woman, but of the free. If the privileges of all believers were so great, according to the new covenant, how absurd for the Gentile converts to be under that law, which could not deliver the unbelieving Jews from bondage or condemnation! We should not have found out this allegory in the history of Sarah and Hagar, if it had not been shown to us, yet we cannot doubt it was intended by the Holy Spirit. It is an explanation of the subject, not an argument in proof of it. The two covenants of works and grace, and legal and evangelical professors, are shadowed forth. Works and fruits brought forth in a man's own strength, are legal. But if arising from faith in Christ, they are evangelical. The first covenant spirit is of bondage unto sin and death. The second covenant spirit is of liberty and freedom; not liberty to sin, but in and unto duty. The first is a spirit of persecution; the second is a spirit of love. Let those professors look to it, who have a violent, harsh, imposing spirit, towards the people of God. Yet as Abraham turned aside to Hagar, so it is possible a believer may turn aside in some things to the covenant of works, when through unbelief and neglect of the promise he acts according to the law, in his own strength; or in a way of violence, not of love, towards the brethren. Yet it is not his way, not his spirit to do so; hence he is never at rest, till he returns to his dependence on Christ again. Let us rest our souls on the Scriptures, and by a gospel hope and cheerful obedience, show that our conversation and treasure are indeed in heaven.

we--The oldest manuscripts and versions are divided between "we" and "ye." "We" better accords with Galatians 4:26, "mother of us."
children of promise--not children after the flesh, but through the promise (Galatians 4:23, Galatians 4:29, Galatians 4:31). "We are" so, and ought to wish to continue so.

Now we - Who believe, whether Jews or Gentiles. Are children of the promise - Not born in a natural way, but by the supernatural power of God. And as such we are heirs of the promise made to believing Abraham.