Paul says here that “an idol is nothing in the world.” Yet the Bible repeatedly condemns idolatry (cf. Ex. 20:4), and even Paul said there are demons behind idols (1 Cor. 10:19). Is he then claiming that demons are nothing? 1 Corinthians 8:4 says, "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." KJV
1 Corinthians 8:4
ESV - 4 Therefore, as to the eating of food offered to idols, we know that "an idol has no real existence," and that "there is no God but one.
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Paul isn't denying that idols exist, but he is denying their ability to affect the true believer.