Religion
The deities of Faerûn are deeply enmeshed in the functioning of the world's magical ecology and the lives of mortals. Faerûnian characters nearly always have a patron deity. Everyone in Faerûn knows that those who die without having a patron deity to escort them to their proper judgment in the land of the dead spend eternity writhing in the Wall of the Faithless, or disappear into the hells of the devils or the infernos of the demons.
The selection of a patron deity does not mean that your character only worships or makes prayers and offerings to one deity. Faerûn is a polytheistic world, not a monotheistic world. At appropriate moments, characters might worship or pay homage to nearly all the deities, even some they could not choose as patron deities. For example, lawful good sailors would never think of choosing Umberlee, the evil goddess of the ocean, as their patron, but it would be hard to find a sailor who had not sacrificed to Umberlee before at least one journey, or made promises to her during a storm. Likewise, an evil follower of Mask, the god of thieves, might make a donation to the temple of Tymora, goddess of luck, before a big heist, even though Tymora is a good goddess.
Why Choose a Patron Deity?
Choosing a patron deity provides you with contacts in the world, particularly if you are known to serve your deity's causes. A character with Helm as her patron is more likely to get assistance -- timely healing, a place of refuge, access to divinations and other spells -- from the church of Helm in times of need. A bard whose patron is Tymora might have a better chance of convincing a group of Tymora-worshiping bandits to talk peacefully instead of fighting.
Of more concern to most adventurers, a character who dies without a patron deity cannot be raised from the dead by any mortal means short of a miracle or wish. When such a character dies, he is considered one of the Faithless, and his soul is used to form part of the wall around the realm of Kelemvor, god of the dead. Mortal action cannot reverse this fate, and so unless the character's friends can arrange direct intervention by another deity (or expend a miracle or wish, spells symbolizing intervention by another deity), that character is unlikely to return to life.
All divine spellcasters (Classes shows which are divine and which are arcane) are granted their divine abilities by a patron deity, so they must choose one.
Choosing a Patron
Having a patron deity implies some true personal attachment to that deity. Given this relationship, it is practically unheard of for a character to have a patron with a radically different alignment than her own. For example, it is essentially impossible for a chaotic good rogue to feel a close personal connection with Bane, the lawful evil god of tyranny and fear.
When choosing a patron if you are a divine spellcaster, you follow the "one-step" rule. Your alignment may be up to one "step" away from your patron's. For example, a chaotic neutral ranger can choose Malar (a chaotic evil god) as his patron, but could not choose Mielikki (a neutral good goddess).
You can only have one patron deity at a time. It is possible to change your patron, but doing so is not a decision made lightly or quickly. If you are a divine spellcaster, [there may be more requirements -- to be determined]. If you are any other character class, changing a patron is a simple matter of deciding to do so that does not require intervention by the church of your new patron (although obtaining its blessing is customary, to show allegiance to the new deity). A character who frequently changes patron deities is likely to gain a reputation of being weak in her faith, and risks being branded as one of the False in the afterlife.
Humans choose a patron deity from the Faerûnian or Mulhorandi pantheons based on the region in which they live or that they grew up in. Nonhumans usually choose a patron from their own pantheon (drow from the drow pantheon, elves from the elven pantheon, and so on). Nonhumans can also select a patron from the human pantheon of the region they live in or grew up in. The most common examples of this are northern halflings, who often choose Tymora, and gnomes, who often choose Gond. Half-orcs choose a patron from the orc pantheon or from the human pantheon of the region they live in or grew up in. Half-elves choose a patron from the elven or drow pantheon (as appropriate to their nonhuman parent) or from the human pantheon of the region they live in or grew up in. For the most part, creatures choose a patron from their own pantheon, but those that stray from this trend are common enough to be viewed merely as a curiosity rather than an aberration.
Next: Deities
Forgotten Realms source material.