Vortex of Mortal Magic
Sorcerers, wizards and bards of the mortal races draw their power from an invisible vortex of energy that permeates the entire world of Avlis, called the Vortex of Mortal Magic. The goddess Andria controls the Vortex of Mortal Magic and thus has the power to determine which individuals have access to it. As the goddess of magic, all mages acknowledge her as being in control and most, though not all, ascribe loyalty to her because she is the source of their power.
Throughout the history of Avlis, strange beings and forces have intermixed with the populace. The ancient Tanar'ri invaders, as well as the native spirits, have left legacies that exist in small pockets where this sort of occurance can be quite common. Occasionally, the blood of these beings resurfaces in one of their descendants who manifests their heritage through the powers of sorcery.
Sorcerers will embrace whatever religions prove worthy in their life experience. Those that gain their sorcerous blood from a living entity, such as a demonlord or deity, will often worship the entity who is responsible for their gift.
At low levels, wizards are allowed to practice magic uninhibited. As they rise in power, they begin to gain the attention of the magical organizations that regulate use of the Vortex. If a wizard allies with one of these organizations, they are granted privileges, incentives, and extra abilities. Conversely, if a wizard chooses to be independent, they will be harassed and possibly even abused for their unwillingness to conform. At certain points in history, they have even had their powers curtailed.
Though wizards may follow any god, all of them acknowledge the divinity of the goddess Andria in particular due to her control of the Vortex of Mortal Magic and her dominion over Arcane Knowledge.