'Howbeit there is not in all men that knowledge, but some, being used until now to the idol, eat as of a thing sacrificed to an idol, and their conscience, being weak, is defiled.'
But not all know fully within their hearts that this is so, that gods are nothing, and that there is but One God. Some still have a superstitious awareness of 'the gods' as though they were 'something' (just as many, even some Christians, have a belief in mascots, talismans and 'luck'). So when they eat of a thing sacrificed to an idol it produces in them superstitious ideas, for the idol has previously been their way of life. It had bound all that they did. Thus they feel when eating food sacrificed to such an idol that in some way they are participating in the god, that it is affecting their lives, that they are becoming involved again, and their consciences are smitten because they consider that they are honouring the god, which they know to be wrong. So by being encouraged by more knowledgeable brothers to partake of food offered to idols, and especially within the temple precincts (1 Corinthians 8:10), they feel compromised and defiled. (To say nothing of the witness before the world). And the result may well be a sinking back into idolatry.
The same can apply to us today. We should avoid all contact with the occult, with fortune-telling, with tarot cards, with seances, and so on, and in our multicultural societies with anything that savours of the worship of gods, because although they may seem nothing to us, those to whom they do mean something will misinterpret our involvement.