But when ye as yet knew not God. This is not intended as an additional argument; and indeed he had already proved his point so fully, that no doubt remained, and the rebuke which was now to be administered could not be evaded. His object is to make their fall appear more criminal, by comparing it with past events. It is not wonderful, he says, that formerly ye did service to them which by nature are no gods; for, wherever ignorance of God exists, there must be dreadful blindness. You were then wandering in darkness, but how disgraceful is it that in the midst of light you should fall into such gross errors! The main inference is, that the Galatians were less excusable for corrupting the gospel than they had formerly been for idolatry. But here it ought to be observed, that, till we have been enlightened in the true knowledge of one God, we always serve idols, whatever pretext we may throw over the false religion. The lawful worship of God, therefore, must be preceded by just views of his character. By nature, that is, in reality, they are no gods. Every object of worship which men contrive is a creature of their own imagination. In the opinion of men idols may be gods, but in reality they are nothing.

Howbeit - But, ἀλλὰ alla. The address in this verse and the following is evidently to the portion of the Galatians who had been pagan. This is probably indicated by the particle ἀλλὰ alla, but denoting a transition. In the previous verses Paul had evidently had the Jewish converts more particularly in his eye, and had described their former condition as one of servitude to the Mosaic rites and customs, and had shown the inconveniences of that condition, compared with the freedom imparted by the gospel. To complete the description, he refers also to the Gentiles, as a condition of worse servitude still, and shows Galatians 4:9 the absurdity of their turning back to a state of bondage of any kind, after the glorious deliverance which they had obtained from the degrading servitude of pagan rites. The sense is, "If the Jews were in such a state of servitude, how much more galling and severe was that of those who had been pagans. Yet fron that servitude the gospel had delivered them, and made them freemen. How absurd now to go back to a state of vassalage, and to become servants under the oppressive rites of the Jewish law!"
When ye knew not God - In your state of paganism, when you had no knowledge of the true God and of his service. The object is not to apologize for what they did, because they did not know God; it is to state the fact that they were in a state of gross and galling servitude.
Ye did service - This does not express the force of the original. The meaning is, "Ye were "slaves" to (ἐδουλεύσατε edouleusate); you were in a condition of servitude, as opposed to the freedom of the gospel;" compare Galatians 4:3, where the same word is used to describe the state of the Jews. The drift of the apostle is, to show that the Jews and Gentiles, before their conversion to Christianity, were in a state of vassalage or servitude, and that it was absurd in the highest degree to return to that condition again.
Unto them which by nature are no gods - Idols, or false gods. The expression "by nature," φύσει phusei, according to Grotius, means, "in fact, re ipsa." The sense is, that they really had no pretensions to divinity. Many of them were imaginary beings; many were the objects of creation, as the sun, and winds, and streams; and many were departed heroes that had been exalted to be objects of worship. Yet the servitude was real. It fettered their faculties; controlled their powers; bound their imagination, and commanded their time and property, and made them slaves. Idolatry is always slavery; and the servitude of sinners to their passions and appetites, to lust and gold, and ambition, is not less galling and severe than was the servitude to the pagan gods or the Jewish rites, or than is the servitude of the African now to a harsh and cruel master. Of all Christians it may be said that before their conversion they "did service," or were slaves to harsh and cruel masters; and nothing but the gospel has made them free. It may be added, that the chains of idolatry all over the world are as fast riveted and as galling as they were in Galatia, and that nothing but the same gospel which Paul preached there can break those chains and restore man to freedom.

When ye knew not God - Though it is evident, from the complexion of the whole of this epistle, that the great body of the Christians in the Churches of Galatia were converts from among the Jews or proselytes to Judaism; yet from this verse it appears that there were some who had been converted from heathenism; unless we suppose that the apostle here particularly addresses those who had been proselytes to Judaism and thence converted to Christianity; which appears to be most likely from the following verses.

(4) Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.
(4) He applies the former doctrine to the Galatians, with a special rebuke: for in comparison with them, the Jews might have pretended some excuse as men that were born and brought up in that service of the Law. But seeing that the Galatians were taken and called out of idolatry to Christian liberty, what pretence might they have to go back to those impotent and beggarly elements?

Howbeit then, when ye know not God,.... Whilst in Gentilism, and in a state of unregeneracy, they had no true knowledge of God; though they might know by the light of nature, and works of creation, that there was a God, yet they did not know who he was, but called either mortal men, or some one or other of the creatures, or stocks, and stones, and images of men's device, by this name; they knew not the God of Israel; they did not know God in Christ, and are therefore said to be without him; and a common description of them it is, that they knew not God: and whilst this was their case, what follows was true of them,
ye did service unto them which by nature are no gods; only by name, and in the opinion of men, but have no divinity in them, are only called gods, mere nominal, fictitious deities, who have nothing of the nature and essence of God in them; for there is but one God by nature and essence, the Father, Son, and Spirit; all others have only the name and appearance, but not the truth of deity; and these the Gentiles in their times of ignorance did "service" to, which is what the Jews call , "strange service"; that is, idolatry, concerning which there is a whole treatise in the Talmud, and which bears that name (o). This service lay in paying homage to them, worshipping of them, and performing various rites and ceremonies in a way of adoration, and which they reckoned religious service; and which, comparatively speaking, whilst in this state of blindness, was excusable in them; though it is a wonderful instance of grace that such idolaters should be the sons of God.
(o) Aveda Zara.

The happy change whereby the Galatians were turned from idols to the living God, and through Christ had received the adoption of sons, was the effect of his free and rich grace; they were laid under the greater obligation to keep to the liberty wherewith he had made them free. All our knowledge of God begins on his part; we know him because we are known of him. Though our religion forbids idolatry, yet many practise spiritual idolatry in their hearts. For what a man loves most, and cares most for, that is his god: some have their riches for their god, some their pleasures, and some their lusts. And many ignorantly worship a god of their own making; a god made all of mercy and no justice. For they persuade themselves that there is mercy for them with God, though they repent not, but go on in their sins. It is possible for those who have made great professions of religion, to be afterwards drawn aside from purity and simplicity. And the more mercy God has shown, in bringing any to know the gospel, and the liberties and privileges of it, the greater their sin and folly in suffering themselves to be deprived of them. Hence all who are members of the outward church should learn to fear and to suspect themselves. We must not be content because we have some good things in ourselves. Paul fears lest his labour is in vain, yet he still labours; and thus to do, whatever follows, is true wisdom and the fear of God. This every man must remember in his place and calling.

Appeal to them not to turn back from their privileges as free sons, to legal bondage again.
then--when ye were "servants" (Galatians 4:7).
ye knew not God--not opposed to Romans 1:21. The heathen originally knew God, as Romans 1:21 states, but did not choose to retain God in their knowledge, and so corrupted the original truth. They might still have known Him, in a measure, from His works, but as a matter of fact they knew Him not, so far as His eternity, His power as the Creator, and His holiness, are concerned.
are no gods--that is, have no existence, such as their worshippers attribute to them, in the nature of things, but only in the corrupt imaginations of their worshippers (see on 1-Corinthians 8:4; 1-Corinthians 10:19-20; 2-Chronicles 13:9). Your "service" was a different bondage from that of the Jews, which was a true service. Yet theirs, like yours, was a burdensome yoke; how then is it ye wish to resume the yoke after that God has transferred both Jews and Gentiles to a free service?

Indeed then when ye knew not God, ye served them that by nature - That is, in reality. Are no gods - And so were under a far worse bondage than even that of the Jews. For they did serve the true God, though in a low, slavish manner.