Greek religion left no sacred book, no creed and no body of clergy authorised to say what was true. That absence is the single most important fact about the evidence, and readers coming from a scriptural tradition consistently underrate it.
What survives instead is literature. Homer and Hesiod supply the genealogies of the gods and most of the stories, and Herodotus already says that these two are the poets who gave the Greeks their gods, which is a remark about literary influence rather than about revelation. Tragedy stages the gods for a civic audience. Philosophers argue against the poets. None of this is doctrine, and where the sources conflict there is no authority to appeal to.
The practice is known from a different set of materials altogether: inscriptions recording sacrifices and dedications, the archaeology of sanctuaries, and the accounts of travellers, of whom Pausanias in the second century AD is the most useful. Cult was local, and a god worshipped in one city could be worshipped quite differently in the next.
The mystery cults are a deliberate gap. Initiates at Eleusis and elsewhere were bound not to speak of what was done, and the silence largely held, so the central rite of one of the most important religious institutions in the Greek world is unknown. Modern reconstructions are inference.
The final filter is transmission. What survives is what medieval Christian scribes chose to copy, and that shaped the canon before any modern scholar saw it.