King James Version
Howbeit there is not in every man that knowledge: for some with conscience of the idol unto this hour eat it as a thing offered unto an idol; and their conscience being weak is defiled.English Standard Version
However, not all possess this knowledge. But some, through former association with idols, eat food as really offered to an idol, and their conscience, being weak, is defiled.New International Version
But not everyone knows this. Some people are still so accustomed to idols that when they eat such food they think of it as having been sacrificed to an idol, and since their conscience is weak, it is defiled.