"When one side goes against the enemy with the gods' gift of stronger morale, then their adversaries, as a rule, cannot withstand them." - Xenophon
In Grepolis there are four gods, Zeus, Poseidon, Hera and Athena. Each god can enact four different divine powers and call upon two unique mythical units.
In the temple you can select which god you would like to worship. Each city can worship different gods, however each city can only worship one god at a time. The higher the level of your Temple, the more favour you earn from your chosen god. You can spend this favour to enact divine powers or call upon mythical units.
What god is right for your city depends on your situation. It is therefore quite useful to change the god you worship many times throughout the game. Do this wisely, for when you switch gods you lose all your favour as well as mythical units and divine powers from your previous god.