And thy brother perish Mark how serious an evil it is, that mankind commonly think so little of -- that of venturing upon anything with a doubtful or opposing conscience. For the object to which our whole life ought to be directed, is the will of the Lord. This, therefore, is the one thing that vitiates all our actions, when we disregard it. [1] This we do, not merely by an outward action, but even by a thought of the mind, when we allow ourselves in anything in opposition to conscience, even though the thing be not evil in itself. Let us bear in mind, therefore, that whenever we take a step in opposition to conscience, we are on the high road to ruin. I read, however, the sentence interrogatively, thus: Shall he perish through thy knowledge? as though he had said: "Is it reasonable that thy knowledge should give occasion of ruin to thy brother? Is it for this reason that thou knowest what is right, that thou mayest cause another's ruin!" He makes use of the term brother, in order to expose their pride as unfeeling, in this way: "It is true that the person whom you despise is weak, but still he is your brother, for God has adopted him. You act a cruel part, therefore, in having no concern for your brother." There is, however, still greater force in what follows -- that even those that are ignorant or weak have been redeemed with the blood of Christ; for nothing were more unseemly than this, that while Christ did not hesitate to die, in order that the weak might not perish, we, on the other hand, reckon as nothing the salvation of those who have been redeemed with so great a price. A memorable saying, by which we are taught how precious the salvation of our brethren ought to be in our esteem, and not merely that of all, but of each individual in particular, inasmuch as the blood of Christ was poured out for each individual!

Footnotes

1 - "Quand nous entreprenons quelque chose center ceste saincte volonte;" -- "When we attempt anything in opposition to that holy will."

And through thy knowledge - Because you knew that an idol was nothing, and that there could be really no danger of falling into idolatry from partaking of these entertainments. You will thus be the means of deceiving and destroying him. The argument of the apostle here is, that if This was to be the result, the duty of those who had this knowledge was plain.
Shall the weak brother - The uninformed and ignorant Christian. That it means real Christian there can be no doubt. Because:
(1) It is the usual term by which Christians are designated - the endearing name of "brother;" and,
(2) The scope of the passage requires it so to be understood; see the note at Romans 14:20.
Perish - Be destroyed; ruined; lost; see the note at John 10:28. So the word ἀπολεῖται apoleitai properly and usually signifies. The sense is, that the tendency of this course would be to lead the weak brother into sin, to apostasy, and to ruin. But this does not prove that any who were truly converted should apostatize and be lost; for:
(1) There may be a tendency to a thing, and yet that thing may never happen. It may be arrested, and the event not occur.
(2) the warning designed to prevent it may be effectual, and be the means of saving. A man in a canoe floating down the Niagara river may have a tendency to go over the falls; but he may be hailed from the shore, and the hailing may be effectual, and he may be saved. The call to him was designed to save him, and actually had that effect. So it may be in the warnings to Christians.
(3) the apostle does not say that any true Christian would be lost. He puts a question; and affirms that if "one" thing was done, "another might" follow. But this is not affirming that anyone would be lost. So I might say that if the man continued to float on toward the falls of Niagara, he would be destroyed. If one thing was done, the other would be a consequence. But this would be very different from a statement that a man "had actually" gone over the falls, and been lost.
(4) it is elsewhere abundantly proved that no one who has been truly converted will apostatize and be destroyed; see the notes at John 10:28; compare the note at Romans 8:29-30.
For whom Christ died - This is urged as an argument why we should not do anything that would tend to destroy the souls of people. And no stronger argument could be used. The argument is, that we should not do anything that would tend to frustrate the work of Christ, that would render the shedding of his blood vain. The possibility of doing this is urged; and that bare possibility should deter us from a course of conduct that might have this tendency. It is an appeal drawn from the deep and tender love, the sufferings, and the dying groans of the Son of God. If He endured so much to save the soul, assuredly we should not pursue a course that would tend to destroy it. If he denied himself so much to redeem, we should not, assuredly, be so fond of self-gratification as to be unwilling to abandon anything that would tend to destroy.

Shall the weak brother perish - Being first taught by thy conduct that there was no harm in thus eating, he grieves the Spirit of God; becomes again darkened and hardened; and, sliding back into idolatry, dies in it, and so finally perishes.
For whom Christ died? - So we learn that a man may perish for whom Christ died: this admits of no quibble. If a man for whom Christ died, apostatizing from Christianity, (for he is called a brother though weak), return again to and die in idolatry, cannot go to heaven; then a man for whom Christ died may perish everlastingly. And if it were possible for a believer, whether strong or weak, to retrace his steps back to idolatry and die in it, surely it is possible for a man, who had escaped the pollutions that are in the world, to return to it, live and die in its spirit, and perish everlastingly also. Let him that readeth understand.

(7) And through thy knowledge shall the weak brother perish, for whom Christ died?
(7) An amplification of the argument taken both of comparison and opposites: "You wretched man", he says, "pleasing yourself with your knowledge which indeed is not knowledge, for if you had true knowledge, you would not sit down to eat food in an idol's temple. Will you destroy your brother, hardening his weak conscience by this example to do evil, for whose salvation Christ himself has died?"

And through thy knowledge,.... These words contain an aggravation of the sin such persons are guilty of, who are the means, by their example, of ensnaring weak minds, and causing them to stumble and fall, even in some sense so as to perish:
shall the weak brother perish, for whom Christ died? every word almost carries in it an exaggeration of this matter; it is not some slight injury that is done to the person, but even causing him to "perish"; and this is not said of any person, but a "brother", to whom the strongest affection, and strictest regard, should be shown; and a "weak" brother, of whom the greater care should be taken; and therefore it is an instance of cruelty to do damage to such an one, and that not ignorantly, which cannot be pretended, but "through thy knowledge"; not through the true use, but abuse of it: those that have knowledge should know better, and improve it to the edification, and not the destruction of fellow Christians; and all this done in a case of indifference, that might as well be let alone, of which there was no necessity for the doing of it: but what aggravates most of all is, that this affects a person for "whom Christ died"; that he had such a value for as to purchase and redeem with the price of his own blood; and yet these men made so little account of, as by so trifling a thing to risk their good and welfare. Some would from hence conclude the doctrine of universal redemption, that Christ died for all men, even for them that perish; but it should be observed, that the words are put by way of interrogation, and prove no matter of fact, even supposing they could be understood of eternal ruin and destruction; and at most only imply the danger and possibility thereof through offences given, were they not preserved by the power and grace of God through Christ, who died for them, and so will not suffer them to perish; though this is no thanks to them who lay stumblingblocks in their way, and, as much as in them lies, cause them to perish, in this sense: besides, the "perishing" of this weak brother is to be understood of his peace and comfort, and is explained by "defiling" his conscience, 1-Corinthians 8:7 by wounding it, 1-Corinthians 8:12 and making him to offend, 1-Corinthians 8:13 through an imprudent use of Christian liberty in those who had the greater knowledge, and by a participation of things offered unto idols, in an idol's temple, and not of his eternal damnation in hell; which could never enter into the apostle's thought, as to be brought about hereby, as appears from 1-Corinthians 8:8 and so is no proof of Christ's dying for such as perish eternally: for those for whom Christ has died, he has by his death procured such blessings for them, as a justifying righteousness, pardon of sin, peace with God, reconciliation unto him, and eternal salvation, which will for ever secure them from perishing in such sense.

shall . . . perish--The oldest manuscripts read "perisheth." A single act seemingly unimportant may produce everlasting consequences. The weak brother loses his faith, and if he do not recover it, his salvation [BENGEL] (Romans 14:23).
for whom Christ died--and for whose sake we too ought to be willing to die (1-John 3:16). And yet professing Christians at Corinth virtually tempted their brethren to their damnation, so far were they from sacrificing aught for their salvation. Note here, that it is no argument against the dogma that Christ died for all, even for those who perish, to say that thus He would have died in vain for many. Scripture is our rule, not our suppositions as to consequences. More is involved in redemption than the salvation of man: the character of God as at once just and loving is vindicated even in the case of the lost for they might have been saved, and so even in their case Christ has not died in vain. So the mercies of God's providence are not in vain, though many abuse them. Even the condemned shall manifest God's love in the great day, in that they too had the offer of God's mercy. It shall be the most awful ingredient in their cup that they might have been saved but would not: Christ died to redeem even them.

And through thy knowledge shall the weak brother perish, for whom Christ died? - And for whom thou wilt not lose a meal's meat, so far from dying for him! We see, Christ died even for them that perish.