Full Idea
Because we have never seen or formed an adequate idea of a god, we picture him to ourselves as a being of the same kind as ourselves but immortal.
Gist of Idea
We cannot conceive of God, so we have to think of Him as an immortal version of ourselves
Source
Plato (Phaedrus [c.368 BCE], 246d)
Book Reference
Plato: 'Phaedrus and Letters VII and VIII', ed/tr. Hamilton,Walter [Penguin 1973], p.51