The topic of gods and the nature of divinity in your campaign are issues a DM may wish to sort out before his campaign begins. Let us break the question into two parts.
Question the First…
What is the actual nature of the gods worshipped and followed by Clerics, Paladins, Druids, and others attuned to the Divine ?
Are the gods entirely mythical ? In your campaign, would you have that the deities named in the Player’s Handbook and expanded upon in other sourcebooks are only personifications of their ascribed ideals and not actual higher beings ? A Cleric’s prayers for her daily allotment of spells may just be a psychological attenuation to a natural force people call “Divine magic”, and not actually the reception of “powers from above”.
Are the gods real entities but exist aloof of the Material Plane ? Perhaps the likes of Kord, Pelor, Erythnul are so far advanced beyond the comprehension of mortals that the gods have no interest or investment in the Material Plane. The gods may have no more notice or concern in mortals’ affairs than mortals may have in the social structure and well-being of an insect colony.
Are the gods active participants in their respective religions ? You may choose that your realm does have a pantheon of gods, all of whom invest time and effort (cooperatively or competitively) in the goings-on in the world of mortals. One might call upon Ehlonna, and be heard by that goddess – the success of such a petition not necessarily guaranteed. If the deities recognize or assist a character may depend on how devout the character has been.
Further even, do the gods manifest themselves ? Here you have a realm described by Homer and the legends of Greek mythology. Gods and goddesses who not only exist and take an active hand in mortal society, but who intercede at their own pleasure. Perhaps by avatar (blue-skinned, or airbending), or in personal disguise, or even in full natural form.
Question the Second…
How do the various religions operate within your campaign’s society ?
Inter-faith relations. Are the followers of one deity friendly, neutral, or hostile towards followers of other deities ? Does the strength or population of some faiths vary by region ? Are some deities elevated to the status of state religion, or perhaps others entirely forbidden ? What reputation has each religion developed in the others’ eyes ?
Internal politics. How has the structure of each deity’s religion evolved over its history in your campaign’s realm ? Is there a loose hierarchy or a rigidly controlled bureaucracy ? What defines the authority structure and how does one ascend the ranks ? Does the religion operate under secretive, mysterious pretenses or is the worship of a deity as transparent and innocuous as it seems to the lay person ? What level of corruption is there within the sect ? Has the faith fragmented into offshoots or remained united and orthodox?
Fraternity. What access and sense of fraternity would a Cleric or Paladin experience if calling upon an unfamiliar church or temple of her religion ? Would a character showing up at the gates of his deity’s temple be met with gracious hospitality and genuine welcome; or would the character be met with suspicion and passive hostility ? How easy is it to become a member of a religion, and what steps are involved ?
Size. Are the churches and temples of a given deity distinct and independent chapter-houses; or ubiquitous franchises dotting the landscape ? Is the authority of a religion centralized or is power concentrated at local levels ? Does the faith survive on tithes and donations or does it benefit from imperial subsidy or wealth benefactors ? Is the faith undergoing an ambitious expansion into new territories or is a deity’s following in a slow, steady decline in the society ?
Role. What role has each religion taken in society ? A modest religion might limit itself to the worship and promotion of the deity and its ideals. A more progressive faith might provide additional services from its temples – medical care, magic spells, divination, perhaps even holding a monopoly on trade goods or owning tracts of farmlands. Or the faith may have moved into the political arena, or provide a significant military presence. Are the religion’s goals practical or lofty, humble or ambitious ?
Besides providing Divine-casting characters with insights into their place in your campaign’s society, such details also add depth and intrigue to your realm.