Divine magic is not necessarily tied to worship of any particular entity. While it is said that particularly powerful saints have occasionally visited miraculous abilities upon their worshippers, most divine magic is performed through an innate connection with the divine and/or with careful study of its properties. There are many priests, but only a handful are capable of true spellcasting.
Outside of the official state religion, many cults, pagan pantheons, and nature worship exists. Worship of demonic entities or other extraplanar forces is also known to happen. Indeed, the Old Magic of the world is considered divine, though it is mostly confined to isolated regions far from the encroachment of civilization.
Clerics, though uncommon overall, are the most plentiful variety of divine spellcaster. Clerics can be found in any form or creed. Some of the priests of the state religion are clerics; some of whom in turn follow specific saints, while others preach the religion in a more holistic sense. Other clerics may be demon-worshippers, cult leaders, or pagan shamans. Some rare few eschew organized religion altogether, and simply tap into the divine spark within themselves without any particular allegiance to a divine figure.
Druids are rarely found within the civilized parts of the Empire. They are practitioners of Old Magic, archaic rituals which most contemporary scholars would consider primitive, but whose power cannot be denied. Powerful druids are rare, but are truly forces to be reckoned with. Druidic magic is the magic of fey creatures and the ancient gods of the land who have long since been forgotten by civilization.
Paladins, like Clerics, have a divine spark that allows them to tap into spiritual magic. They are from the same stock, but Paladins are those who have chosen to focus their energies down the path of the righteous warrior. Crusaders against evil and militant hospitallers are examples of Paladins, and several knightly orders exist whose ranks include many such individuals. Like Clerics, Paladins may or may not follow a specific saint or belief system, although Paladins by their very nature are more likely to do so.
Rangers are warriors who have tapped into the Old Ways. They work with magic in a similar manner to Druids, but rely less on formal ritual and more on their own innate divine energies and knowledge of Old Magic and its gods and spirits. Because of this, they tend to be much less powerful spellcasters than Druids, but make up for it with their martial skills.