Henceforward he appears to digress into a moral discourse, but in a new manner, which does not occur in any other of his Epistles. For all of them are divided into two parts, and in the first he discusses doctrine, in the last the rule of life, but here, after having entered upon the moral discourse, he again unites with it the doctrinal part. For this passage has reference to doctrine in the controversy with the Manichees. What is the meaning of, “Use not your freedom for an occasion to the flesh?” Christ has delivered us, he says, from the yoke of bondage, He has left us free to act as we will, not that we may use our liberty for evil, but that we may have ground for receiving a higher reward, advancing to a higher philosophy. Lest any one should suspect, from his calling the Law over and over again a yoke of bondage, and a bringing on of the curse, that his object in enjoining an abandonment of the Law, was that one might live lawlessly, he corrects this notion, and states his object to be, not that our course of life might be lawless, but that our philosophy might surpass the Law. For the bonds of the Law are broken, and I say this not that our standard may be lowered, but that it may be exalted. For both he who commits fornication, and he who leads a virgin life, pass the bounds of the Law, but not in the same direction; the one is led away to the worse, the other is elevated to the better; the one transgresses the Law, the other transcends it. Thus Paul says that Christ has removed the yoke from you, not that you may prance and kick, but that though without the yoke you may proceed at a well-measured pace. And next he shows the mode whereby this may be readily effected; and what is this mode? He says,
Ver. 13. “But through love be servants one to another.”
Here again he hints that strife and party-spirit, love of rule and presumptousness, had been the causes of their error, for the desire of rule is the mother of heresies. By saying, “Be servants one to another,” he shows that the evil had arisen from this presumptuous and arrogant spirit, and therefore he applies a corresponding remedy. As your divisions arose from your desire to domineer over each other, “serve one another;” thus will you be reconciled again. However, he does not openly express their fault, but he openly tells them its corrective, that through this they may become aware of that; as if one were not to tell an immodest person of his immodesty, but were continually to exhort him to chastity. He that loves his neighbor as he ought, declines not to be servant to him more humbly than any servant. As fire, brought into contact with wax, easily softens it, so does the warmth of love dissolve all arrogance and presumption more powerfully than fire. Wherefore he says not, “love one another,” merely, but, “be servants one to another,” thus signifying the intensity of the affection. When the yoke of the Law was taken off them that they might not caper off and away another was laid on, that of love, stronger than the former, yet far lighter and pleasanter; and, to point out the way to obey it, he adds;
Ver. 14. “For the whole law is fulfilled in one word, even in this; You shall love your neighbor as yourself.”
Seeing that they made so much of the Law, he says, “If you wish to fulfill it, do not be circumcised, for it is fulfilled not in circumcision but in love.” Observe how he cannot forget his grief, but constantly touches upon what troubled him, even when launched into his moral discourse.
Ver. 15. “But if you bite and devour one another, take heed that you be not consumed one of another.”
That he may not distress them, he does not assert this, though he knew it was the case, but mentions it ambiguously. For he does not say, “Inasmuch as you bite one another,” nor again does he assert, in the clause following, that they shall be consumed by each other; but “take heed that you be not consumed one of another,” and this is the language of apprehension and warning, not of condemnation. And the words which he uses are expressly significant; he says not merely, “you bite,” which one might do in a passion, but also “you devour,” which implies a bearing of malice. To bite is to satisfy the feeling of anger, but to devour is a proof of the most savage ferocity. The biting and devouring he speaks of are not bodily, but of a much more cruel kind; for it is not such an injury to taste the flesh of man, as to fix one's fangs in his soul. In proportion as the soul is more precious than the body, is damage to it more serious. “Take heed that you be not consumed one of another.” For those who commit injury and lay plots, do so in order to destroy others; therefore he says, Take heed that this evil fall not on your own heads. For strife and dissensions are the ruin and destruction as well of those who admit as of those who introduce them, and eats out every thing worse than a moth does.
Ver. 16. “But I say, Walk by the Spirit, and you shall not fulfil the lust of the flesh.”
Here he points out another path which makes duty easy, and secures what had been said, a path whereby love is generated, and which is fenced in by love. For nothing, nothing I say, renders us so susceptible of love, as to be spiritual, and nothing is such an inducement to the Spirit to abide in us, as the strength of love. Therefore he says, “Walk by the Spirit and you shall not fulfil the lust of the flesh:” having spoken of the cause of the disease, he likewise mentions the remedy which confers health. And what is this, what is the destruction of the evils we have spoken of, but the life in the Spirit? Hence he says, “Walk by the Spirit and you shall not fulfil the lust of the flesh.”
Ver. 17. “For the flesh lusts against the Spirit, and the Spirit against the flesh, for these are contrary the one to the other: that you may not do the things that you would.”
Source: Commentary on Galatians (New Advent)