Faerunian clerics function as described in the Players Handbook except that no clerics serve just a cause, philosophy, or abstract source of divine power. The Torilian deities are very real, and events in recent history have forced these divine beings to pay a great deal of attention to their mortal followers.
All clerics in Faerun serve a patron deity. (In fact, most people in Faerun choose a deity as their patron.) It is simply impossible for a person to gain divine powers (such as divine spells) without one. You may not have more than one patron deity at a time, although, it is possible to change your patron deity if you have a change of heart. You cannot multiclass into another class that requires a patron deity unless your previous patron deity is an acceptable choice for the new class. For example, you cannot multiclass as a druid unless your patron deity is a nature deity (since all druids have nature deities as patron deities).