How does Paul use the status of an underage heir and the allegory of Abraham's two sons to urge the Galatians to reject legal servitude?

Paul explains that so long as an heir remains an underage minor, his actual situation differs in no way from an enslaved servant, even though he is the legal owner of the entire estate. Instead, he remains subject to tutors and guardians until the date predetermined by the father. In like manner, during their spiritual infancy, believers were held in servitude beneath the rudimentary elements of the world. Challoner's annotation explains that serving under the elements refers either to the introductory rudiments of religion wherein carnal Jews were trained, or to corporeal creatures employed in manifold ritual sacrifices, rites, and sacraments. When the fullness of the designated time arrived, God sent forth His divine Son, born of a woman and placed under the Mosaic law, to ransom those living under legal subjection so that believers might receive full adoption as sons. Because believers are authentic sons, God sent the Spirit of His Son into their hearts, crying out: Abba, Father. Consequently, the believer is no longer an enslaved servant but a true son, and being a son, an heir through God.

Reflecting on their prior condition, Paul reminds the Galatians that before knowing God, they were enslaved to beings that by nature were not gods at all. Having now come to know God—or rather being known and claimed by God—he wonders how they can regress to weak and impoverished elements, desiring to serve them anew. They scrupulously observe calendar days, months, seasons, and years. Challoner clarifies that the apostle does not condemn the Christian observance of Sunday or sacred liturgical feasts, but rather targets either superstitious observation of lucky and unlucky days, or Jewish festival calendars that Judaizing teachers sought to impose. Paul voices grave apprehension over them, anxious lest all his strenuous labors in their midst might prove wasted.

With heartfelt personal warmth, Paul implores the Galatians to become like him, since he had adapted himself to them, reminding them that they had done him no injury. They remember that he preached the gospel to them through infirmity of the flesh, yet despite the trial his physical ailment posed, they neither showed contempt nor rejected him, welcoming him as an angel of God, even as Christ Jesus Himself. He asks where their former sense of blessedness has vanished, testifying that if it were possible, they would have gouged out their own eyes to give them to him. Wondering if he has become their adversary simply by telling the truth, Paul warns that rival teachers court them with misguided zeal, seeking to isolate them so that the converts will show zeal toward their party. True zeal is commendable always in a worthy cause, not merely when Paul is present. Addressing them affectionately as his little children, he endures spiritual birth pangs once again until Christ is fully formed within them, wishing he were present in person to alter the tone of his voice because he is ashamed for them.

Challenging those who desire to remain under legal servitude, Paul asks whether they truly listen to the law, citing the narrative of Abraham's two sons, one born of an enslaved bondwoman and the other of a free woman. The child of the bondwoman was born according to ordinary fleshly nature, whereas the child of the free woman arrived by virtue of divine promise. Paul interprets these accounts allegorically as representing two covenants: the one stemming from Mount Sinai engenders children into bondage, corresponding to Hagar. Mount Sinai in Arabia corresponds to the present earthly Jerusalem, which abides in servitude alongside her children. In contrast, the celestial Jerusalem above is free and is our mother, fulfilling the prophetic text that commands the barren woman who never bore to rejoice and shout aloud, because the desolate woman has more offspring than the wife who possesses a husband. Believers, like Isaac, are the children of promise. Just as the son born according to the flesh once persecuted the son born according to the Spirit, the same hostility persists today. Yet scripture commands the expulsion of the bondwoman and her son, decreeing that the bondwoman's child shall never inherit alongside the son of the free woman. Brethren are therefore not children of the bondwoman but sons of the free woman, through the freedom wherewith Christ has liberated us.