'What do I say then? That a thing sacrificed to idols is anything, or that an idol is anything? But I say, that the things which they sacrifice, they sacrifice to demons, and not to God. And I do not wish that you should be sharers in common with demons.'
He firmly insists that he is not by this saying that a thing sacrificed to an idol is anything special, or that an idol is anything special. What he is saying is that in fact idolatrous worship is not just harmless superstition, it is backed by demons, by evil spirits, and that whoever offer sacrifices to idols, whether Israel in its false worship of the molten calf, or Gentiles in the worship of idols, are thus unknowingly offering sacrifices to demons (compare Deuteronomy 32:17). They are not to be seen as worshipping God in any way. Their way is not just another way to God, it provides contact with the supernatural world of evil. So what Paul is encouraging them to avoid is to actually have things in common with 'the demons', that is, the whole world of demons.