The same approach may be followed in the case of private invitations to a meal. In the example given, the invitation comes from an unbeliever.
Christian witness is paramount. You are a believer and, in the example, he is an unbeliever. Your whole life is about witnessing to Christ. You want to win him to salvation. You don’t want to put any obstacle in his way. How can you witness to an idol worshipper and tell him that Christ requires him to turn away completely from his idols, when at the same time you are undermining your witness by eating food offered to an idol? He will not understand that. He will not be able to process that. His current level of knowledge tells him that becoming a Christian means stopping any contact with idolatry, including eating food offered to idols which has been a big part of his life. He is not yet ready to take the extra step and say, an idol is nothing. Don’t hurry him faster than he is capable of going. This has now become a matter of conscience. The same principle applies to other matters where the unbeliever makes scruples of matters which are of no real consequence.
Why does Paul use precisely the same reason for now doing the opposite? Before he was happy for the one purchasing food in the meat market to eat, because ‘the earth is the Lord’s, and the fullness thereof.’ Now he tells the guest not to eat, because ‘the earth is the Lord’s, and the fullness thereof.’ Most commentators point out that these words are not found in a great many Greek texts. Gill explains them by saying that if the believer avoids eating meat offered to idols, then God has made so many alternative foods that he will not suffer any shortage by abstaining.