(Shar battles Selune)
Across all of Toril, people respect and fear the divine powers. The deities of Faerun take an active role in the world, promoting the causes they favor, watching over the domain for which they are responsible, and constantly seeking to increase (or at least defend) their temporal power by protecting their worshipers and encouraging the active expansion of their faiths.
Mortals who deny the deities who made the world and govern its basic forces are rare indeed, altough a few powerful beings such as the enigmatic sharns and phaerimms acknowledge no entity as their superior. Human (and humanoid) souls who refuse the gods come to a bad end after death, lacking a deity to speak for them upon the Fugue Plane. What befalls primal creatures such as the sharns, no once can say.
Some Faerunians zealously follow one deity. Others make sacrifices to many deities, while upholding one of their personal patron. Still others sacrifice to as many deities as possible, shifting allegiances as their circumstances and needs warrant. It's a rare Faerunian who hasn't occasionally hoped to avert the baleful influence of an evil deity with a propitious gift, or thanked a good power for an unexpected blessing. The belief system of most Faerunians generally centers on a particular deity whose interests and influence are most likely to affect them, but acknowledges other gods as significant and important, too.
The deities of Toril take an active interest in their world, channeling power through their clerics, druids, rangers, paladins, and other worshipers and sometimes intervening directly in the affairs of mortals. At the same time, they plot war, intrigue, and ally among themselves, between themselves and powerful mortals, and with extraplanar beings such as elemental rulers and demons. IN this they resemble their mortal worshipers, for to an extent deities are defined and shaped by their worshipers, their areas of interest, and their nature – for many deities are actually mortals who have gained the divine spark. Because the lose strength if their worship dwindles away and is forgotten, deities task their clerics and others to whom they grant divine spells with spreading their praise and doctrine, recruiting new worshipers, and keeping the faith alive. In exchange for this work and to facilitate it, deities grant divine spells.
Worshipers
A weaponsmith might take Gond as his patron deity, but also pray to Tempus, Lord of Battles, before attempting to forge a fine sword. During a difficult forging or when striving to make a blade lucky for wielders, the same smith prays to Tymora. A weapon forged for guardians would involve prayers and offerings to Helm. A weapon to be wielded for justice (an executioner's blade, perhaps) would be dedicated to Tyr.
Most people of Toril worship more than one deity on a daily basis, even if they dedicate their lives to one patron deity. Some folk of Faerun believe deities are akin to awesomely powerful mortals and are therefore prone to foibles, tempers, and the haste, mistakes, and emotions of mortals. Others see them as beyond mortal flaws or mortal comprehension. Overlaid on these extreme beliefs as to whether deities like to intervene in mortal affairs daily, at crucial junctures, on whims, or to further mysterious or stated aims – or whether they remain aloof, influencing mortals only in subtle, hidden ways or through dream visions or cryptic auguries. With there widely varying views come a correspondingly wide range in practices and worship.
With that said, many folk make offerings both to deities they revere and appeasement offerings to deities of markedly different alignment and interests from their own to ward off holy vengefulness, spite, and divine whim. The simplest offering to a deity is to toss a few coins into a temple bowl or make another suitable offering (blood to Tempus or Malar, for example, or particular sacred or token objects to most other deities) while a pleas is murmured. The formalization of this practice is the payment of a set temple fee to clergy of the deity to be appeased, who either provide the payer with a short prayer to be performed at an auspicious later time or perform a rote prayer for the payer.
Patron Deities
The deities of Faerun are deeply enmeshed in the functioning of the world's magical ecology and the lives of mortals. Faerunian characters nearly always have a patron deity. Everyone in Faerun 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.
Sins and Penance
Some members of the clergy believe their deities watch over every act, thought, and consequence of the deeds of every mortal worshiper. Most priests, however, see their deities as judging mortals only on deeds or on acts plus obvious intent rather than ultimate consequences. A cleric or druid who commits a minor offense against her deity or ignores portions of the deity's dogma is guilty of a sin. He has to do some penance appropriate to the seriousness of the sin in order to remain in good standing with the church, other clerics or druids, and the deity. Paladins, rangers and other divine spellcasters are held to this standard (to a less exacting degree) also.
Typical penance for lesser infractions includes spending an hour in prayer, making a small monetary donation to the temple, performing minor duties in the temple and so on.
Penance for moderate infractions include spending anywhere from a day to a tendency in prayer, making a moderate monetary donation to the temple, or going on a small quest for the church.
Penance for major infractions includes a month or more of prayer, a large donation, a quest, and possibly an atonement spell (which might require its own quest).
Continued abuses of the church's dogma may result in a divine spellcaster losing his class features (but not any class related weapon and armor proficiencies) until he atones for his sins.
Changing Deities
It is possible for a cleric, druid, paladin, or spellcasting ranger (or any other divine spellcaster) to abandon his chosen deity and take up the faith of another deity. In doing so, the divine spellcaster loses all class features of the abandoned deity. To progress as a divine spellcaster of another faith, the character must go on a quest for his new church (often the recovery of a lost item of some importance to the deity), then receive an atonement spell from a representative of his new faith. Once these two conditions are met, the character becomes a divine spellcaster of the new deity, and if a cleric, he chooses two domains from the new deity's repertoire. The character then resumes the class features lost from leaving the old faith (So long as they are still applicable - turning or rebuking undead ability might change for instance)
Note: DM approval and intervention is required for changing your deity.
Religions of Sanctuary
No one religion holds a grip on the Underdark, as most of its inhabitants worship their own racial pantheon. This especially holds true in the town of Sanctuary, where the ex-slaves seeking refuge here bring with them their own religions. Though a few temples and shrines have been established within its walls, it is a melting pot of religions from the various races and regions found throughout Toril.