1 Corinthians 8:7
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. 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.
Not every person has the knowledge that an idol is nothing, so some still regard food offered to an idol as genuinely connected with idol worship. When those with a weak conscience eat such food as something offered to an idol, their weak conscience is defiled. In that setting, even buying such food could appear to sanction idolatrous worship.