Piety in Godwars is a special non-tradable currency only obtained through difficult measures such as the Arena or defeating Dimension Bosses.
You can use your piety from the God Powers→Use Piety menu. The more piety you use at the Altar, the more return you get on each dice of rewards with a final report on the total effects. Piety can currently be used to gain bestowable experience, finances or more action points. Action points will automatically stop being used if you reach your cap.