context: how humans first accepted eloquence and poetry
haec eloquentiae primordia, haec penetralia; hoc primum habitu cultuque commoda mortalibus in illa casta et nullis contacta vitiis pectora influxit: sic oracula loquebantur.
Translation: These are the first-beginnings of eloquence, these the inner sanctum. With this dress and allure, enticing to mortals, eloquence infused simple hearts, as yet untouched by vices: to such the oracles used to speak.
Problems:
commoda . . . mortalibus: commoda, I read as agreeing with plural primordia, but I am troubled by the singular subject of_influxit_, which I think implies eloquentia.
Re commoda modifying understood eloquentia: I considered that reading, but laid it aside out of timidity. That definitely makes the logic more consistent.
Re ablative of manner: right! It seems so obvious now.