Polytheism is the belief in many gods, although as you have read, there is but One God dealing with the Known Lands. However, this does not make idolatry impossible. Idol worship and pagan worship is worship in a polytheistic sense. Their are two polytheistic movements in the Known Lands: Oerthism and Olympianism.
The original Polytheistic model of the Known Lands until the arrival of the Bretonia in Ancient Times was Oerthism (also known as the D&D pantheon). Oerthistic clerics still exist, but they are pretty much underground when other religions such as the Path of Light, the Holy Light, and Christianity has taken prominence. Oerthists do not have places of worship any more except for hidden temples.
The Deities worshipped include:
- Corellon Larethian
- St. Cuthbert
- Wee Jas
Olympianism has come to the Known Lands with the arrival of the Bretonia. It's a pagan idol religion that comes from the people of Hellas, or the Ancient Greeks. The pantheon is tight, but worship is underground. Most Olympian clerics all look to Zeus, although the others are often worshipped. The pantheon includes: