On the Mysteries (De Mysteriis)

Written as a reply to a sceptical letter from Porphyry: if the gods are impassive and beyond persuasion, what can a rite possibly do? Iamblichus answers that the rite does not persuade anyone. It works because the divine has already sown tokens of itself in matter, names and sounds, and the practitioner is joining a descent that was offered first.

The founding defence of ritual as participation rather than compulsion — the oldest surviving statement of the boundary this app keeps.