9 What need of more, Brethren. You are Christians, and have heard, that “If you sin against the brethren, and wound their weak conscience, you sin against Christ.” Do not disregard it, if you would not be wiped out of the book of life. How long shall I go about to speak in bright and pleasing terms to you, what my grief forces me to speak in some sort, and will not suffer me to keep secret? Whosoever they are who are minded to disregard these things, and sin against Christ, let them only consider what they are doing.
We wish the rest of the Heathen to be gathered in; and you are stones in their way: they have a wish to come; they stumble, and so return. For they say in their hearts, Why should we leave the gods whom the very Christians worship as we do? God forbid, you will say, that I should worship the gods of the Gentiles. I know, I understand, I believe you. But what account are you making of the consciences of the weak which you are wounding? What account are you making of their price, if you disregard the purchase?
Consider for how great a price was the purchase made. “Through your knowledge,” says the Apostle, “shall the weak brother perish;” that knowledge which you profess to have, in that you know that an idol is nothing, and that in your mind you are thinking only of God, and so sittest down in the idol's temple. In this knowledge the weak brother perishes. And lest you should pay no regard to the weak brother, he added, “for whom Christ died.” If you would disregard him, yet consider his Price, and weigh the whole world in the balance with the Blood of Christ.
And lest you should still think that you are sinning against a weak brother, and so esteem it after that he had heard that he was “Peter,” a trivial fault, and of small account, he says, “You sin against Christ.” For men are in the habit of saying, I sin against man; am I sinning against God? Deny then that Christ is God. Do you dare deny that Christ is God? Have you learned this other doctrine, when you sat at meat in the idol's temple? The school of Christ does not admit that doctrine.
I ask; Where did you learn that Christ is not God? The Pagans are wont to say so. Do you see what bad associations do? Do you see, “that evil communications corrupt good manners?” There you can not speak of the Gospel, and you hear others talking of idols. There you lose the truth that Christ is God; and what you drink in there, you vomit out in the Church. It may be you are bold enough to speak here; bold enough to mutter among the crowds; “Was not then Christ a man? Was He not crucified?”
This have you learned of the Pagans. You have lost your soul's health, you have not touched the border. On this point then touch again the border, and receive health. As I taught you to touch it in this that is written, “Whoso sees a brother sit at meat in the idol's temple;” touch it also concerning the Divinity of Christ. The same border said of the Jews, “Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever.” Behold, against Whom, even the Very God, you sin, when you sit down with false gods.
Source: Sermons on Selected Lessons of the New Testament (New Advent)