The Metamorphoses, a history of the world with an emphasis on the part played by the Greek gods, is generally considered to be Ovid's magnum opus. He's most famous either for that poem or for his love poetry. But some people think very highly of his Fasti, which goes through the Roman calendar of festivals month by month and concentrates, more than does the Metamorphoses, on Rome's home-grown deities, and less on the Greek imports. Ovid completed half of the poem, going half of the way through the year. Here are the first lines of Book 6:
Hic quoque mensis habet dubias in nomine causas:
quae placeat, positis omnibus ipse leges.
facta canam; sed erunt qui me finxisse loquantur,
nullaque mortali numina visa putent.
est deus in nobis, agitante calescimus illo;
impetus hic sacrae semina mentis habet:
fas mihi praecipue voltus vidisse deorum,
vel quia sum vates, vel quia sacra cano.
(It's not certain whence came the name of the month of June. I'll list them all, choose the one you like. I'll sing the truth, but some will claim I'm lying and that no-one ever saw any gods. A god is in us. We're warmed when he stirs, and it's his impulse which inspires us. I'm allowed to see the faces of the god, whether it's because I'm a poet or because I sing of the sacred.)
I would not be doing my proper atheist duty if I failed to balance this rather sincere- and literal-seeming expression of theistic belief by mentioning that in Ars Amatoria Ovid wrote the much less literal-sounding
Expedit esse deos, et, ut expedit, esse putemus.
(It's convenient that there are gods, and since it's convenient, let's believe it.)
As for the Metamorphoses and the Fasti: it's more convenient to act as if the gods are real when they're your main subject. Whatever Ovid believed, I believe his poems are smashing.