The apostle, having established the consolatory doctrine that believers, in every age and country of the world, are heirs of the promises made to Abraham and to his seed, goes on in this chapter to answer an inquiry which he knew would naturally occur to his readers, but which, according to his manner, he does not formally state; namely, Since all believers, from the beginning, were heirs of the promises, as well as of the things promised, why were they not put in possession of the promises from the beginning, by sending Christ into the world, and introducing the gospel dispensation in the first age; that the promises, especially the promise of pardon and eternal life through faith, might have been published universally, and preserved for the benefit of the heirs in every age; and why were mankind left for so many ages to the direction of the laws of nature and of Moses, neither of which gave them any hope of pardon and eternal life? To this inquiry the apostle answers, that in not giving the heirs the knowledge of the promises, by introducing the gospel dispensation immediately after the fall, God treated them as a prudent father treats his son while under age. During his nonage, he does not allow him to possess the estate, of which he is the heir, because he has not discretion to use it aright; but keeps him in the condition of a bond-man. In the same manner, though believers from the beginning were heirs of the promises, God did not, in the early ages, put them in possession of them, by immediately setting up the gospel dispensation; because, in the first ages, the state of the world did not admit of either the universal publication of the gospel, or of its preservation. And that, as the heir of a great estate must be prepared by a proper education for managing and enjoying it, and is therefore in his childhood placed under persons who instruct him, manage his estate, and supply him with necessaries, till the time appointed in his father's will for taking possession of his inheritance; so, to prepare believers for the actual inheritance of the gospel dispensation, God judged it proper to continue them for a long time under the bondage of the laws of nature, and of the patriarchal and Mosaic dispensations, that by experiencing the hardships of that bondage, they might be the more sensible of the happiness which they were to derive from the liberty of the gospel. This is the sense of the three first verses, as appears by the following short paraphrase.
Now To illustrate, by a plain similitude, the pre-eminence of the Christian over the legal dispensation; I say that the heir Of any estate, however large; as long as he is a child Or is under age; differeth nothing from a servant With respect to the free use and enjoyment of his estate; though he be lord of all Proprietor of it all, by right of inheritance; but is placed under tutors As to his person; and governors Οικονομους, stewards, as to his substance; until the time appointed of the father When he shall be deemed of age, and be at liberty to manage his affairs himself. So we The church of God, heirs of the promises; when we were children In our minority, were not put in possession of the promises, by the introduction of the gospel dispensation, but, to fit us for it, were placed in bondage In a kind of servile state; under the elements of the world Under the typical observances of the patriarchal and Mosaic dispensations, which were like the first elements of grammar, the ABC of children; and were of so gross a nature as hardly to carry men's thoughts beyond this world. Seeing the apostle, in the close of the preceding chapter, declared that all who have put on Christ, (see on Galatians 4:27-29,) whether they be Jews or Gentiles, are Christ's brethren, and heirs according to God's promise, “it is evident that in this chapter, when he speaks concerning the heir, and describes the treatment which, by his father's appointment, he receives during his minority, his discourse cannot be restricted to the Jews, as if they were the only heirs, but must comprehend the Gentiles also, describing their condition under the discipline of the law of nature, Galatians 4:8. In like manner the persons in bondage to the elements of the world, (Galatians 4:3,) and under the law, (Galatians 4:5,) who are said to be bought off by Christ, (Galatians 4:5,) must be the Gentiles as well as the Jews; because Jews and Gentiles equally were under the discipline [and curse] of law. And having been bought off by Christ, (Galatians 3:13,) they were both of them, after his death, placed under the gospel dispensation, which is the discipline of sons. See Galatians 4:5.” Macknight.