I know that is a broad question, but I would just like some examples of what the beliefs (gods, myths, ceremonies, etc.) were of tribal groups in Europe before they were all converted to Christianity.
As a quick note, not all of the people's in Europe were tribal (pre-state societies) before the spread of Christianity. The Latins lived in a unitary (quasi-nation) state, and Greeks lived in various state societies.
The germanic tribes had the germanic religion which had a pantheon of gods including Thor and Odin. As an example of a ceremony, Yule was originally a germanic festival before it was subsumed as part of the winter ceremonies in Christianity. It originally occurred in midwinter after the last harvest, and involved a lot of feasting to celebrate the period. It was a significant time in the germanic calendar, so much so that two months were named in reference to it among the west germanic speakers who inhabited Britain during and after the fall of the roman empire (December = Ærra Gēola "before yule", January = Æfterra Gēola "after Jule").
Followup:
What evidence do we have for Odinn and Thor worship before the Viking Age?