1 Corinthians 8:4
As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one.
Idols are senseless and possess no real divine power, since there is only one God. There is no other God besides the one true God. Where food had been offered in sacrifice to idols, buying that food for eating could be seen as giving support to idolatrous worship.