The difficulty arises because the distinction between fact and fiction seems very clear today (because writers and publishers assign texts to genres that are factual or fictional but not both), but travel back in time, as we are asked to do by any question about the earliest example of some trope, and the distinction gradually becomes harder and harder to make. Fact becomes mixed with opinion and theology and folklore that is partly invention, while fiction becomes mixed with tradition and legend and myth that is partly historical. The Iliad probably preserves some facts about the Trojan War, and the epic of Gilgamesh about Uruk, while Pliny and Herodotus probably contain many fictions, but we have no way to be sure exactly which parts are which. Look at this answer for how much research one can do and still not know for sure what Homer meant when he wrote “Ithaca”!
And in any case, the nature of the text may preclude any simple fact/fiction determination. Take something like Cicero's On Divination. This is fiction because it's written in the form of a dialogue between Cicero and his brother Quintus that they (almost certainly) never spoke. But it's also fact because Cicero is recording (probably accurately) common Roman beliefs about divination. But it's also fiction because you can't predict the future by looking at the entrails of sacrificial animals.
Because the Bible is the longest text surviving from antiquity, and because there are so many works of Western literature that were influenced by it, it is inevitable that early examples of many tropes in fiction are going to be found there. Whatever the precise mixture of fact and fiction in the Bible, it still influenced many later writers, and from the literary point of view it is rarely useful to make a determination either way: we can explain a biblical allusion without having to take a stance on whether the passage alluded to is fact or fiction.
Accordingly, my suggestion is that if we spot a question that relies on distinguishing fact from fiction in many works of literature, especially if it is asking about early examples of some trope, we encourage the OP to revise the question so that it no longer depends on this distinction, referring them here (or to some suitable FAQ) to explain why insisting on this distinction is unlikely to be productive when looking back to ancient literature. This will save us from the impossible task of having to make a fact/fiction determination on all the works under consideration (which, depending on the question, might potentially be all works ever written!), and so avoid contention. If someone does want to ask about the extent to which a particular text is factual, that that’s fine: it has a low risk of becoming contentious since the asker must be aware that there is some doubt about the answer.