The Iliad is a religious text, though not in the way we might understand that term today. The text is permeated with the supernatural and communicates forcefully that the gods—or perhaps even some power above the gods—are in charge of the fate and ruin of humans.
There is a supernatural world that supersedes humankind by being more powerful and far seeing, and yet at the same time this spirit world is very close at hand. One of the surprising elements of the poem, in fact, is the way the gods are always nearby. They may be invisible to humans, but they are present, sometimes in the corner of a scene, directing and controlling events. They are active participants in the here and now of the Trojan war.
For example, the entire Trojan war is a result of three quarreling goddesses dragging a human, Paris, in to settle their fight. Later, Apollo sends a plague to the Greeks when Agamemnon refuses to return Chryseis to her father in Thebe. The gods intervene to slow Achilles down when he embarks on a killing rampage against the Trojans. The gods decide when it is Hector's appointed time to die, so they deceive him to fighting Achilles.
Ultimately, Troy comes to ruin not because of the lack of skill of the Trojans but because this ruin is fated. The poem conveys the moral message central to Greek thought that humans cannot escape what is predestined.