The Iliad, in roughly 15,700 lines, takes a narrow slice of the Trojan war: Agamemnon's seizure of Achilles' prize, the withdrawal of Achilles from the fighting, the death of Patroklos in his armour, the killing of Hektor, and finally Priam coming by night to ransom his son's body. The Odyssey, some 12,100 lines, splits between the wanderings, told largely as Odysseus' own retrospective narration at the Phaeacian court, and the slower business on Ithaca of testing loyalties and destroying the suitors.
For the study of Greek religion the poems matter less for doctrine than for portraiture. The Olympians appear as a household with a hierarchy and grievances. They take sides, deceive each other, are wounded, sulk, and are constrained by Zeus and, at moments, by fate. Divine intervention is constant but psychologically continuous with human motive: a god puts courage into a warrior, and the warrior is still the one acting.
The poems also record practice. Sacrifice is described in procedural detail, from the sprinkled barley to the thigh bones wrapped in fat, along with libation, prayer formulae, oath-taking, supplication, prophecy from birds and entrails, funeral rites and cremation, and the horror attached to a corpse left unburied. Much of what is known about how Greeks approached the gods before the classical period comes from these descriptions, embedded in narrative rather than set out as instruction.
The two poems do not agree about death. The Iliad's dead are shades without strength, and Achilles in the Odyssey tells Odysseus he would rather be a hired labourer alive than king among the dead. Yet the same book of the Odyssey shows Minos judging and Tantalus and Sisyphus punished, and elsewhere Menelaus is promised the Elysian plain rather than death. These are not reconciled, and the poems were never expected to reconcile them.