Let us now see on what grounds Celsus urges us to make use of the idol offerings and the public sacrifices in the public feasts. His words are, “If these idols are nothing, what harm will there be in taking part in the feast? On the other hand, if they are demons, it is certain that they too are God's creatures, and that we must believe in them, sacrifice to them according to the laws, and pray to them that they may be propitious.” In reference to this statement, it would be profitable for us to take up and clearly explain the whole passage of the first Epistle to the Corinthians, in which Paul treats of offerings to idols. The apostle draws from the fact that “an idol is nothing in the world,” the consequence that it is injurious to use things offered to idols; and he shows to those who have ears to hear on such subjects, that he who partakes of things offered to idols is worse than a murderer, for he destroys his own brethren, for whom Christ died.
And further, he maintains that the sacrifices are made to demons; and from that he proceeds to show that those who join the table of demons become associated with the demons; and he concludes that a man cannot both be a partaker of the table of the Lord and of the table of demons. But since it would require a whole treatise to set forth fully all that is contained on this subject in the Epistle to the Corinthians, we shall content ourselves with this brief statement of the argument; for it will be evident to any one who carefully considers what has been said, that even if idols are nothing, nevertheless it is an awful thing to join in idol festivals.
And even supposing that there are such beings as demons to whom the sacrifices are offered, it has been clearly shown that we are forbidden to take part in these festivals, when we know the difference between the table of the Lord and the table of demons. And knowing this, we endeavour as much as we can to be always partakers of the Lord's table, and beware to the utmost of joining at any time the table of demons.
Source: Against Celsus (New Advent)