[Tennessee Ernie Ford]

Piety directly affects spell points in each realm. Increase your piety during level up and observe. Of all the things you do to grow SPs, Piety has the strongest affect.

Piety also leads to IronWill, which adds resistance to spells (something the low HP professions, like mages, will find useful).

Piety affects Divinity by making it 'easier' to learn. (PCs w/high divinity grow divinity skill faster than those w/low stat.) A few other skills also use piety, but I think the effect can be safely ignored for non-Priests.

Piety also affects HPs, Carrying Capacity, and Stamina, about half as much as Vitality and Strength do.

I recommend that mages focus on INT/PIE or INT/(PIE/SPD). Wizards run out of SPs too fast. They're also handicapped by low HP, CC, and Stamina.