The apostle is here dealing with those who, having embraced the faith of Christ, still continued to seek for justification by the works of the law; that is, who depended upon their own obedience to the moral precepts as their righteousness before God, and, wherein that was defective, had recourse to the legal sacrifices and purifications to make it up. These he first sharply reproves, and then endeavours, by the evidence of truth, to convince them. This is the right method, when we reprove any for a fault or an error, to convince them that it is an error, that it is a fault.
He reproves them, and the reproof is very close and warm: he calls them foolish Galatians, Gal 3:1. Though as Christians they were Wisdom's children, yet as corrupt Christians they were foolish children. Yea, he asks, Who hath bewitched you? whereby he represents them as enchanted by the arts and snares of their seducing teachers, and so far deluded as to act very unlike themselves. That wherein their folly and infatuation appeared was that they did not obey the truth; that is, they did not adhere to the gospel way of justification, wherein they had been taught, and which they had professed to embrace. Note, It is not enough to know the truth, and to say we believe it, but we must obey it too; we must heartily submit to it, and stedfastly abide by it. Note, also, Those are spiritually bewitched who, when the truth as it is in Jesus is plainly set before them, will not thus obey it. Several things proved and aggravated the folly of these Christians.
1. Jesus Christ had been evidently set forth as crucified among them; that is, they had had the doctrine of the cross preached to them, and the sacrament of the Lord's supper administered among them, in both which Christ crucified had been set before them. Now, it was the greatest madness that could be for those who had acquaintance with such sacred mysteries, and admittance to such great solemnities, not to obey the truth which was thus published to them, and signed and sealed in that ordinance. Note, The consideration of the honours and privileges we have been admitted to as Christians should shame us out of the folly of apostasy and backsliding.
2. He appeals to the experiences they had had of the working of the Spirit upon their souls (Gal 3:2); he puts them in mind that, upon their becoming Christians, they had received the Spirit, that many of them at least had been made partakers not only of the sanctifying influences, but of the miraculous gifts, of the Holy Spirit, which were eminent proofs of the truth of the Christian religion and the several doctrines of it, and especially of this, that justification is by Christ only, and not by the works of the law, which was one of the peculiar and fundamental principles of it. To convince them of the folly of their departing from this doctrine, he desires to know how they came by these gifts and graces: Was it by the works of the law, that is, the preaching of the necessity of these in order to justification? This they could not say, for that doctrine had not then been preached to them, nor had they, as Gentiles, any pretence to justification in that way. Or was it by the hearing of faith, that is, the preaching of the doctrine of faith in Christ as the only way of justification? This, if they would say the truth, they were obliged to own, and therefore must be very unreasonable if they should reject a doctrine of the good effects of which they had had such experience. Note, (1.) It is usually by the ministry of the gospel that the Spirit is communicated to persons. And, (2.) Those are very unwise who suffer themselves to be turned away from the ministry and doctrine which have been blessed to their spiritual advantage.
3. He calls upon them to consider their past and present conduct, and thence to judge whether they were not acting very weakly and unreasonably (Gal 3:3, Gal 3:4): he tells them that they had begun in the Spirit, but now were seeking to be made perfect by the flesh; they had embraced the doctrine of the gospel, by means of which they had received the Spirit, and wherein only the true way of justification is revealed. And thus they had begun well; but now they were turning to the law, and expected to be advanced to higher degrees of perfection by adding the observance of it to faith in Christ, in order to their justification, which could end in nothing but their shame and disappointment: for this, instead of being an improvement upon the gospel, was really a perversion of it; and, while they sought to be justified in this way, they were so far from being more perfect Christians that they were more in danger of becoming no Christians at all; hereby they were pulling down with one hand what they had built with the other, and undoing what they had hitherto done in Christianity. Yea, he further puts them in mind that they had not only embraced the Christian doctrine, but suffered for it too; and therefore their folly would be the more aggravated, if now they should desert it: for in this case all that they had suffered would be in vain - it would appear that they had been foolish in suffering for what they now deserted, and their sufferings would be altogether in vain, and of no advantage to them. Note, (1.) It is the folly of apostates that they lose the benefit of all they have done in religion, or suffered for it. And, (2.) It is very sad for any to live in an age of services and sufferings, of sabbaths, sermons, and sacraments, in vain; in this case former righteousness shall not be mentioned.
4. He puts them in mind that they had had ministers among them (and particularly himself) who came with a divine seal and commission; for they had ministered the Spirit to them, and wrought miracles among them: and he appeals to them whether they did it by the works of the law or by the hearing of faith, whether the doctrine that was preached by them, and confirmed by the miraculous gifts and operations of the Spirit, was that of justification by the works of the law or by the faith of Christ; they very well knew that it was not the former, but the latter; and therefore must needs be inexcusable in forsaking a doctrine which had been so signally owned and attested, and exchanging it for one that had received no such attestations.

Having begun in the Spirit - Having received a spiritual religion, which refined and purified your hearts; and having received the Holy Spirit of God, by which ye were endued with various miraculous influences; and the spirit of adoption, by which he were assured of the remission of sins, and incorporation with the family of God:
Are ye now made perfect by the flesh? - Are ye seeking to complete that spiritual religion, and to perfect these spiritual gifts, by the carnal rite of circumcision? It appears that by the Spirit, here, not only the Holy Spirit, but his gifts, are to be understood; and by the flesh, illud membrum in quo circumcisio peragitur; and, by a metonymy, circumcision itself.

Are ye so foolish?.... Is it possible you should be so stupid? and do you, or can you continue so?

having begun in the Spirit; that is, either in the Spirit of God, whom they had received through the preaching of the Gospel. They set out in a profession of religion in the light, under the influence, and by the assistance of the Spirit; they began to worship the Lord in spirit, and in truth, without any confidence in the flesh; they entered upon the service of God, and a newness of life, a different conversation than before, a spiritual way of living in a dependence on the grace and help of the divine Spirit: or in the Gospel, which is the Spirit that gives life, is the ministration of the Spirit of God, and contains spiritual doctrines, and gives an account of spiritual blessings, and is attended with the Holy Ghost, and with power. This was first preached unto them, and they embraced it; this they begun and set out with in their Christian profession, and yet it looked as if they sought to end with something else:

are you now made perfect by the flesh? or "in" it; not in carnality, in the lusts of the flesh, as if they now walked and lived after the flesh, in a carnal, dissolute, wicked course of life; for the apostle is not charging them with immoralities, but complaining of their principles: wherefore, by "the flesh" is meant, either the strength of mere nature, in opposition to the Spirit of God, by which they endeavoured to perform obedience to the law; or else the law itself, in distinction from the Gospel; and particularly the ceremonial law, the law of a carnal commandment, and which consisted of carnal ordinances, and only sanctified to the purifying of the flesh; and also their obedience to it; yea, even all their own righteousness, the best of it, which is but flesh, merely external, weak, and insufficient to justify before God. This is a third aggravation of their folly, that whereas they begun their Christian race depending upon the Spirit and grace of God, now they seemed to be taking a step as if they thought to finish it in the mere strength of nature; and whereas they set out with the clear Gospel of Christ, and sought for justification only by his righteousness, they were now verging to the law, and seeking to make their justifying righteousness perfect, by joining the works of the law unto it, which needed them not, but was perfect without them.

begun--the Christian life (Phi 1:6).

in the Spirit--Not merely was Christ crucified "graphically set forth" in my preaching, but also "the Spirit" confirmed the word preached, by imparting His spiritual gifts. "Having thus begun" with the receiving His spiritual gifts, "are ye now being made perfect" (so the Greek), that is, are ye seeking to be made perfect with "fleshly" ordinances of the law? [ESTIUS]. Compare Rom 2:28; Phi 3:3; Heb 9:10. Having begun in the Spirit, that is, the Holy Spirit ruling your spiritual life as its "essence and active principle" [ELLICOTT], in contrast to "the flesh," the element in which the law works [ALFORD]. Having begun your Christianity in the Spirit, that is, in the divine life that proceeds from faith, are ye seeking after something higher still (the perfecting of your Christianity) in the sensuous and the earthly, which cannot possibly elevate the inner life of the Spirit, namely, outward ceremonies? [NEANDER]. No doubt the Galatians thought that they were going more deeply into the Spirit; for the flesh may be easily mistaken for the Spirit, even by those who have made progress, unless they continue to maintain a pure faith [BENGEL].