an idol worshipper: a person who gives a false god the service that belongs to God alone
Greek built this word by joining the word for an idol (εἴδωλον) to the word for the service a worshipper renders to a god (λατρεύω). So it does not describe someone holding wrong opinions about God. It describes someone who shows up: who eats at the table of a false god, joins its festival, and hands over the honor God asks for himself. Notice too that it names a kind of person, not a practice, which is why it turns up in lists of people rather than lists of behaviors.
Six of its seven appearances sit in exactly such a list. Paul sets it beside the sexually immoral, the greedy, and swindlers when he tells the Corinthian church whom it may and may not sit down to eat with, and again when he names the kinds of people who will not inherit God's kingdom. Revelation repeats the pattern twice, leaving idol worshippers outside the city with murderers, liars, and those who deal in magic. In 1 Corinthians 10:7 the word becomes a warning instead of a label: do not become what Israel became at the golden calf, where eating, drinking, and playing were the whole of the worship.
The seventh use bends the word onto a target no one expects. Ephesians 5:5 says the greedy person is an idol worshipper, and Colossians 3:5 makes the same move with the word for the practice (εἰδωλολατρεία). This is not a softening of the charge into a figure of speech. It is the claim that a settled hunger for more behaves like a god: it is served, it is trusted, and it collects the life that belongs to God.