Pretty much everyone knows God created everything originally. There's more debate about just how active He/She is in the World now. Some believe the Almighty is ever-present. Some consider God much more distant. However, the power of Clerics, Paladins, and such definitely comes from somewhere, and it certainly seems to be God.
Also, the existence of the Celestial Host is undeniable (by all but the most uneducated or isolated of peoples.) Powerful celestial entities (known by most mortals as Angels) are occasionally active in various places in the World, and these Angels certainly speak of God as a singular, all-powerful, entity of pure goodness who nevertheless supports the free will of mortals in the World.
Mortal religion is mostly divided into Churches of various Saints. Certain extraordinary mortal, celestial, or mixed mortal/celestial individuals (Half-celestials, Aasimars, etc.) who were influential in the World gained Church followings. The theology, practices, and preferences of each of these Church groups reflect the life of the Saint they revere. Different Churches honoring different Saints coexist more-or-less peacefully, sometimes joining forces to combat evil cults and such.
Fiends of various power levels also exist and sometimes do great damage in the World. It is supposed that the powers/spells/abilities of cultists and evil clerics (who are most definitely illegal pretty much everywhere law exists) come from the most powerful of these Fiendish entities.
Finally, there also exist various nature spirits and such, who are occasionally worshiped by tribal peoples and other remote groups.