Litavism

Litavism is the ethnic religion of Lithuania, the only country in Europe to never have officially converted to Christianity (beyond brief periods for political reasons). Litavism is a polytheistic religion centered around various Indo-European gods.

The name Litavism is derived from the German name for Lithuania, Litauen. A term came to be used by German writers noting the Lithuanians during the Baltic Crusade, Liatuismus, which was used as a term for the culture as a whole. It later came to denote their polytheistic religion specifically (compare the origin of the term Hindu). The Lithuanians themselves refer to their religion as Dievųgarbinimas, meaning Worship of the Gods.  

Lithuania never officially converted to Christianity, and no attempts were ever made to Christianize the locals. The final rejection of Christianity came in 1327 when the Duke of Lithuania, Gediminas, outlawed Christianity in Lithuania, with punishment by death. Gediminas had secured an alliance with Russia shortly before, as well as with The Kingdom of Mirga, and managed to utilize this to keep the Christians away from war with Lithuania. The full might of the Pagan Empire was shown when, with the help of Russia and the Romani, Lithuania destroyed and conquered all land belonging to the Baltic Germanic Monastic states. Lithuania went to war with Poland shortly after, managing to just narrowly defeat the Poles.

While Poland would later conquer the Romani states, They never managed to fully conquer Lithuania, whose alliance to Russia provided it with a powerful non-Christian ally in Europe and provided Russia a friendly buffer against Christian incursions into its territory.