A.T. Robertson Commentary 1 Corinthians 8:4

"Concerning therefore the eating of things sacrificed to idols, we know that no idol is [anything] in the world, and that there is no God but one." — 1 Corinthians 8:4 (ASV)

No idol is anything in the world (ουδεν ειδωλον εν κοσμω). Probably correct translation, though no copula is expressed. On ειδωλον (from ειδος), old word, see on Ac 7:41; 15:20; 1 Thessalonians 1:9. The idol was a mere picture or symbol of a god. If the god has no existence, the idol is a non-entity. This Gentile Christians had come to know as Jews and Jewish Christians already knew.

No God but one (ουδεις θεος ε μη εις). This Christians held as firmly as Jews. The worship of Jesus as God's Son and the Holy Spirit does not recognize three Gods, but one God in three Persons. It was the worship of Mary the Mother of Jesus that gave Mahomet his cry: "Allah is One." The cosmos, the ordered universe, can only be ruled by one God (Romans 1:20).