Some timely and pertinent comments from Dr. Luther's 1535 Lectures on Galatians (CPH 1963):
"‘What does the Scripture say? Cast out the slave and her son; for the son of the slave shall not inherit with the son of the free woman’ (Gal. 4:30).
"Here the Ishmaelites hear a sentence pronounced against them that overthrows the Jews, the Greeks, the Romans, etc., who persecuted the church of Christ. It will also overthrow the papists and all other self-righteous people, whoever they may be, who boast today that they are the people of God and the church; they hope that they will surely receive the inheritance, and they judge that we who depend on the promise of God are not only barren and desolate but heretics who have been cast out of the church and cannot be sons and heirs. But God turns this judgment of theirs upside down and pronounces the sentence on them that because they are sons of the slave woman and persecute the sons of the free woman, they are to be cast out of the house and are not to share the inheritance with the sons of the promise, to whom, as sons of the free woman, the promise belongs exclusively. This sentence is legitimate, and it is irrevocable; for ‘Scripture cannot be broken’ (John 10:35). Therefore it will most certainly happen that our Ishmaelites will not only lose the authority they have in both church and state but eternal life as well. For Scripture has foretold that the sons of the slave woman are to be cast out of the house, that is, out of the kingdom of grace, because they cannot inherit with the sons of the free woman.
"Now it should be noted that the Holy Spirit insults the people of the Law and of works here by calling them ‘sons of the slave woman.’ It is as though He were to say: ‘Why do you boast about the righteousness of the Law and of works and brag that on its account you are the people and the children of God? If you do not know whence you have your birth, I will tell you. You were born as slaves and of a slave woman. What sort of slaves? Of the Law, sin, death, and eternal damnation. A slave is not an heir but is cast out of the house.’ Therefore the pope, with all his regime and whatever other self-righteous people there are, regardless of how saintly they may be in appearance, who trust they will obtain grace and salvation through laws human or divine — all of them are sons of the slave woman who will not inherit with the sons of the free woman but will be cast out of the house. And I am not speaking now about the ungodly monks, who worshiped their belly as god (Phil. 3:19) and committed horrible sins that I would just as soon not mention, but about the best of them, to whom I and many others belonged, who lived holy lives and tried with might and main to appease the wrath of God and to merit the forgiveness of sins and eternal life by the observances of their religious order. These men must now hear the sentence that the sons of the slave woman are to be cast out of the house together with the slave woman, their mother.
"When they are carefully considered, such sentences provide us with certainty and reassurance regarding the doctrine and the righteousness of faith, in opposition to the doctrine and the righteousness of works. The world embraces and praises the latter, and it despises and condemns the former. This, of course, is disturbing and offensive to timid souls, who, even when they see the wickedness and the unspeakable crimes of the papists publicly exposed, cannot be easily persuaded that the whole multitude bearing the name and title of ‘the church’ are in error and that only a few believe correctly about the doctrine of faith. If the papacy still had the sanctity and austerity of life that it had at the time of fathers like Jerome, Ambrose, Augustine, and others, when the clergy did not yet have an evil reputation for simony, extravagance, pleasures, wealth, adultery, sodomy, and countless other sins but lived in accordance with the canons and decrees of the fathers, outwardly religious and holy, and even practiced celibacy — what would we have been able to do against the papacy?
"Celibacy, which the clergy observed rigorously at the time of the fathers, is a remarkable thing in the eyes of the world, a thing that makes a man into an angel. Hence Paul calls it ‘religion of angels’ (Col. 2:18), and the papists sing about virgins: ‘He led an angelic life in the flesh, for he lived beyond the flesh.’ And the so-called ‘contemplative life,’ to which the clergy were very devoted then at the cost of all their civic and domestic responsibilities, also presents an impressive front of sanctity. Therefore if the outward appearance of the ancient papacy were still standing, we would accomplish very little against it with our doctrine about faith, especially because we are accomplishing very little against it now, when that primitive reputation for piety and strict discipline has vanished and when all that is evident in the papacy are the dregs and bilge of vice.
"But even if the religion and discipline of the papacy stood now as it did once, we would still have to follow the example of Paul, who attacked the false apostles despite their holy and virtuous fronts, and battle against the self-righteousness of the papal kingdom, saying: ‘Regardless of how celibate a life you lead or how you conduct yourselves in humility and the religion of angels or how you wear out your bodies with frequent discipline, you are salves of the Law, of sin, and of the devil; and you will be cast out of the house, because you seek righteousness and salvation through your own works, not through Christ.’
"Therefore we should pay attention not so much to the sinful lives of the papists as to their wicked doctrine and their hypocrisy, and this is what we chiefly attack. Let us suppose that the religion and the discipline of the ancient papacy were flourishing now and being observed with the same rigor with which the hermits, Jerome, Augustine, Gregory, Bernard, Francis, Dominic, and many others observed it. We would still have to say: ‘If you have nothing to set against the wrath and judgment of God except your sanctity and the chastity of your lives, you are clearly sons of the slave woman, who must be cast out of the kingdom of heaven and condemned.’
"Satan does not defend the wicked lives of the papists either — which the more wholesome among them also despise — but he fights to defend and preserve their hypocrisy and works-righteousness. Here he cites the authority of the councils and the examples of the holy fathers, whom he declares to have been the founders of the holy orders and statutes. Therefore we are fighting today, not against the obvious wickedness and vice of the papacy but against its fictitious saints, who think that they lead an angelic life when they observe not only the commandments of God but also the counsels of Christ and works that are not required or works of supererogation. We say that this is a waste of time and effort, unless they have grasped that ‘one thing’ which Christ says is the only thing ‘needful’ and, like Mary, have chosen the good portion, which cannot be taken away from them (Luke 10:42).
"That is what Bernard did. He was a man so pious, holy, and chaste that I think he deserves to be put ahead of all other monks. Once, when he was gravely ill and despaired of his life, he did not place his trust in the celibacy that he had observed so chastely, or in the good works and acts of piety that he had performed in such quantity; but he put all these far from sight and took hold of the blessing of Christ by faith, saying: ‘I have lived damnably. But Thou, O Lord Jesus Christ, hast a double right to the kingdom of heaven: first, because Thou art the Son of God; secondly, because Thou hast won it by Thy passion and death. The first Thou dost keep for Thyself by Thy birthright; the latter Thou dost grant to me by the right, not of works but of grace.’ He did not set his monkery or his angelic life against the wrath and judgment of God but took hold of the one thing that is needful and thus was saved. I believe that Jerome, Gregory, and many other fathers and hermits were saved the same way. There is no doubt that in the Old Testament also many kings of Israel and other idolaters were saved in a similar way, casting away their vain trust in idols at the hour of their death and taking hold of the promise of God regarding the future Offspring of Abraham, the Christ, who was to bless all nations. And if any of the papists are to be saved today, they must depend, not on their own good works and merits but solely on the mercy of God offered to us in Christ; and they must say with Paul: ‘I do not have a righteousness of my own, based on Law, but that which is through faith in Christ’ (Phil. 3:9).
"‘So, brethren, we are not children of the slave but of the free woman’ (Galatians 4:31). Here Paul concludes the allegory. ‘We are not,’ he says, ‘children of the slave.’ That is: We are not under the Law, which gives birth into slavery, which terrifies, accuses, and leads to despair. But we are free of it through Christ; therefore it cannot terrify and condemn us, as we have pointed out in detail earlier. Moreover, regardless of how much the children of the slave woman persecute us for a time, our comfort is still that they will finally be thrown into the outer darkness (Matt. 8:12) and be forced to leave to us the inheritance that belongs to us as children of the free woman.
"As we have heard, Paul found in the words ‘children of the free woman and of the slave’ a wonderful opportunity to argue in support of the doctrine of justification. And he deliberately took hold of this word ‘free’ and urged and developed it also in what follows. From it he took the occasion to discuss Christian liberty, the knowledge of which is extremely necessary. For the pope has completely destroyed it and has subjected the church to a miserable and bitter slavery by means of human traditions and ceremonies. The liberty that has been granted to us through Christ is today our chief defense against the tyranny of the pope. Therefore the doctrine of Christian liberty must be carefully considered, both to support the doctrine of justification and to encourage and comfort our consciences against the many disturbances and offenses which, as our opponents claim, have arisen from the Gospel. Now Christian liberty is a completely spiritual matter. The unspiritual man does not understand it (1 Cor. 2:14). In fact, even those who have the first fruits of the Spirit (Rom. 8:23) and who can discourse about it at great length have difficulty keeping it in their hearts. To reason it appears to be a matter of little importance. Therefore unless the Spirit magnifies it and adds weight to it, it will be despised.
"As he approaches the end of the epistle, Paul argues vigorously and passionately in defense of the doctrine of faith and of Christian liberty against the false apostles, who are its enemies and destroyers. He aims and hurls veritable thunderbolts of words at them to lay them low. At the same time he urges the Galatians to avoid their wicked doctrine as though it were some sort of plague. In the course of his urging he threatens, promises, and tries every device to keep them in the freedom achieved for them by Christ. Therefore he says:
"‘For freedom Christ has set us free; stand fast therefore’ (Galatians 5:1). That is: ‘Be firm!’ Thus Peter says: ‘Be sober, be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour. Resist him, firm in your faith’ (1 Peter 5:8–9). ‘Do not be smug,’ he says, ‘but be firm. Do not lie down or sleep, but stand.’ It is as though he were saying: ‘Vigilance and steadiness are necessary if you are to keep the freedom for which Christ has set us free. Those who are smug and sleepy are not able to keep it.’ For Satan violently hates the light of the Gospel, that is, the teaching of grace, freedom, comfort, and life. Therefore as soon as he sees it arise, he immediately strives to obliterate it with all his winds and storms. For this reason Paul urges godly persons not to be drowsy and smug in their behavior but to stand bravely in the battle against Satan, lest he take away the freedom achieved for them by Christ." (Luther’s Works, Volumes 26–27, CPH 1963; alt.)