The Greek philosopher Epicurus apparently said this:
“Is God willing to prevent evil, but not able? Then he is not omnipotent.Is he able, but not willing? Then he is malevolent.Is he both able and willing? Then whence cometh evil?Is he neither able nor willing? Then why call him God?”
The Greeks at the time believed in the ancient pantheon, not the Christian God, so under what context could he have referred to a single, omnipotent god?
The short answer is that whilst this is called the Epicurean Paradox, the quote can’t actually be attributed to Epicurus, but instead to later (monotheistic) philosophers/scholars .
Epicurus himself lived in the 3rd/4th century BC and moved between a variety of Greek cities before settling in Athens. Whilst he is said to have authored many works, none of those have actually survived to the present bar a few fragments, quotes and letters.
Most of the principles we call Epicureanism are therefore written by later Epicurean scholars who probably had access to these texts, most famously the Roman poet Lucretius (1st century BC) and Greek historian Diogenes Laertius (3rd century AD).
Whilst it is entirely possible that Epicurus did indeed author the paradox, it is only first attributed to him by Lactantius (3rd/4th century AD) who himself was a Christian. In other words, the oldest version we have of the paradox is written by a monotheist. This version was then quoted by later philosophers, particularly in the enlightenment by Hume and Kant. Another argument is that the paradox is not even Epicurean but instead comes from the Skeptics. Regardless of which argument you believe, the conclusion is that the paradox as we know it doesn’t appear until a time significantly after Epicurus’s death. A time when monotheism in the Roman Empire was becoming more prominent.
How plausible it is whether the paradox is Epicurean is is up for debate. As you noted the Greeks did not believe their gods to be omnipotent or infallible, so the text would look a bit odd if you simply changed god for gods, particularly from the modern context that this is a refutation of omniscience. The point of Epicureanism is not however to refute the existence of god (or gods), but to state that if these beings do exist in some shape or form, they are irrelevant to every day life. From that angle the problem could still be applied to a polytheistic society.
I don’t know about the original wording of this specific passage but I strongly suspect this answer! to a similar question written by /u/Iphikrates answers your question.