"You can not drink the cup of the Lord and the cup of demons ; you can not participate in the Lord's table and the table of demons." 1 Corinthians 10. 21
In ancient times, everyday life was steeped in religion : it intervened in all social events.
The notions of freedom of conscience and secularism did not exist; not to revere the gods was a crime, for it was to despise the protection they were supposed to give to the city or country that honored them. It is understandable why the Jews first and the Christians afterwards, who did not participate in the many pagan rites, were so violently persecuted. This interweaving of the sacred and the profane created many difficulties for Christians. Paul had to answer a question : can a Christian eat the meat of sacrificed animals in honor of a god ?
The sacrifices were followed by a meal taken in common, and the meat of the animal sacrificed to a deity was eaten. It ranged from a simple meal to orgies where all sorts of impure things were mixed together. The pressure was strong on Christians to participate in these social as well as religious events.
Idols in themselves are nothing but matter, but any participation in an idolatrous rite is a communion with the powers of evil that hide behind the idol. This is why Paul intervenes firmly to declare that no compromise is possible between the domain of darkness and that of the divine light. Believers had to break with all idolatrous manifestations. Through their participation in the Lord's Supper, they had communion with his body given for them, with his blood shed for them. How could they also participate in the idolatrous meals that put them in communion with the demons, with the power of Satan ?
This question no longer arises in this form for Christians in Western countries. But it is very current, in Africa or Asia, for many brothers and sisters from animist families. For them, it is often a very difficult choice : they want to be submitted to the authorities, to show love and respect ; but they can not be present where a spirit is invoked, where one sacrifices to a power related to Satan.
Source (Pleasing the Lord)