Not by faith, but as it were by works, etc. As false zeal seems commonly to be justly excused, Paul shows that they are deservedly rejected, who attempt to attain salvation by trusting in their own works; for they, as far as they can, abolish faith, without which no salvation can be expected. Hence, were they to gain their object, such a success would be the annihilation of true righteousness. You farther see how faith and the merits of works are contrasted, as things altogether contrary to each other. As then trust in works is the chief hinderance, by which our way to obtain righteousness is closed up, it is necessary that we should wholly renounce it in order that we may depend on God's goodness alone. This example of the Jews ought indeed justly to terrify all those who strive to obtain the kingdom of God by works. Nor does he understand by the works of the law, ceremonial observances, as it has been before shown, but the merits of those works to which faith is opposed, which looks, as I may say, with both eyes on the mercy of God alone, without casting one glance on any worthiness of its own. For they have stumbled at the stone, etc. He confirms by a strong reason the preceding sentence. There is indeed nothing more inconsistent than that they should obtain righteousness who strive to destroy it. Christ has been given to us for righteousness, whosoever obtrudes on God the righteousness of works, attempts to rob him of his own office. And hence it appears that whenever men, under the empty pretence of being zealous for righteousness, put confidence in their works, they do in their furious madness carry on war with God himself. But how they stumble at Christ, who trust in their works, it is not difficult to understand; for except we own ourselves to be sinners, void and destitute of any righteousness of our own, we obscure the dignity of Christ, which consists in this, that to us all he is light, life, resurrection, righteousness, and healing. But how is he all these things, except that he illuminates the blind, restores the lost, quickens the dead, raises up those who are reduced to nothing, cleanses those who are full of filth, cures and heals those infected with diseases? Nay, when we claim for ourselves any righteousness we in a manner contend with the power of Christ; for his office is no less to beat down all the pride of the flesh, than to relieve and comfort those who labour and are wearied under their burden. The quotation is rightly made; for God in that passage declares that he would be to the people of Judah and of Israel for a rock of offence, at which they should stumble and fall. Since Christ is that God who spoke by the Prophets, it is no wonder that this also should be fulfilled in him. And by calling Christ the stone of stumbling, he reminds us that it is not to be wondered at if they made no progress in the way of righteousness, who through their wilful stubbornness stumbled at the rock of offence, when God had showed to them the way so plainly. [1] But we must observe, that this stumbling does not properly belong to Christ viewed in himself; but, on the contrary, it is what happens through the wickedness of men, according to what immediately follows.

Footnotes

1 - "Error is often a greater obstacle to the salvation of men than carelessness or vice... Let no man think error in doctrine a slight practical evil. No road to perdition has ever been more thronged than that of false doctrine. Error is a shield over the conscience and a bandage over the eyes." -- Professor Hodge

Wherefore? - Why? The apostle proceeds to state the reason why so uniform and remarkable a result happened. "They sought it not by faith, etc." They depended on their own righteousness, and not on the mercy of God to be obtained by faith.
By the works of the law - By complying with all the demands of the Law so that they might merit salvation. Their attempted obedience included their prayers, fastings, sacrifices, etc., as well as compliance with the demands of the moral law. It may be asked here, perhaps, how the Jews could know any better than this? how should they know anything about justification by faith? To this I answer:
(1) That the doctrine was stated in the Old Testament; see Habakkuk 2:4; compare Romans 1:17; Psalm 32:1-11; Psalm 130:1-8; Psalm 14:1-7; compare Romans. 3; Job 9:2.
(2) the sacrifices had reference to a future state of things, and were doubt less so understood; see the Epistle to the Hebrews.
(3) the "principle" of justification, and of living by faith, had been fully brought out in the lives and experience of the saints of old; see Romans. 4 and Hebrews. 11.
They stumbled - They fell; or failed; or "this was the cause why they" did not obtain it.
At that stumbling-stone - To wit, at what he specifies in the following verse. "A stumbling-stone" is a stone or impediment in the path over which people may fall. Here it means "that obstacle which prevented their attaining the righteousness of faith; and which was the occasion of their fall, rejection, and ruin." That was the rejection and the crucifixion of their own Messiah; their unwillingness to be saved by him; their contempt of him and his message. For this God withheld from them the blessings of justification, and was about to cast them off as a people. This also the apostle proceeds to prove was foretold by the prophets.

Wherefore? - And where lies their mistake? Being ignorant of God's righteousness - of his method of saving sinners by faith in Christ, they went about to establish their own righteousness - their own method of obtaining everlasting salvation. They attend not to the Abrahamic covenant, which stands on the extensive principles of grace and faith; but they turn all their regards to the law of Moses. They imagine that their obedience to that law gives them a right to the blessings of the Messiah's kingdom. But, finding that the Gospel sets our special interest in God and the privileges of his Church on a different footing, they are offended, and refuse to come into it.

Wherefore? Because [they sought it] not by faith, but as it were by the (s) works of the law. For they stumbled at that stumblingstone;
(s) Seeking to attain righteousness, they followed the law of righteousness.

Wherefore? because they sought it not by faith,.... The question is asked, why they did not attain to that, which with so much diligence they pressed after? the answer is, because, as they did not seek for righteousness in a right place, or object, they sought for it in the law, and the works of it, where it is never to be found by a sinful creature, and not in Christ, in whom only are righteousness and strength; so they did not seek for it in a right way, by faith in Christ, without which it is impossible to please God, and by which only true righteousness is discerned and received:
but as it were by the works of the law; not by works which looked like works of the law, and were not; but they sought it as if they expected their justification before God was to be by works of righteousness done by them; or as if it was partly by their own works, and partly by the goodness of God, accepting of them for a justifying righteousness. The Alexandrian Copy, and some others, read only, "as it were by works"; and so does the Vulgate Latin version: another reason, or else a reason of the former is,
for they stumbled at that stumbling stone; meaning the word of the Gospel, at which Peter says they stumbled, and particularly the doctrine of justification by the righteousness of Christ; or rather Christ himself, who was "to the Jews a stumbling block, and to the Greeks foolishness", 1-Corinthians 1:23.

Wherefore? Because they sought it not by faith, but as it were--rather simply, "as"
by the works of the law--as if it were thus attainable, which justification is not: Since, therefore, it is attainable only by faith, they missed it.
for--it is doubtful if this particle was originally in the text.
they stumbled at that stumbling-stone--better, "against the stone of stumbling," meaning Christ. But in this they only did.

And wherefore have they not? Is it because God eternally decreed they should not? There is nothing like this to be met with but agreeable to his argument the apostle gives us this good reason for it, Because they sought it not by faith - Whereby alone it could be attained. But as it were - In effect, if not professsedly, by works. For they stumbled at that stumblingstone - Christ crucified.