Assuming Poland can be considered as Northern Europe. ;-)

A short summary is below, for more information check here: http://www.krakowpost.com/article/6956

Polish Paganizm

As with most agricultural societies, Slavs for many years worshipped the Great Gods and Goddess which were represented in earliest times by the Bird, Bee, Snake Goddess, and then with the outside influence by Mother Earth. Their year was separated into two main divisions, Yule to Summer, ruled by the White God and Summer to Yule, ruled by the Black Goddess. They celebrated in the open air around trees that were particularly old or had peculiar and special significance. Public ritual and celebration, feasts, prophecies and offerings accompanied all rites. In Poland they were accustomed to raising their hand towards the sun when they swore an oath. The peasants greeted the rise of the solar star with a deep bow and a prayer.

The Polish term for god is bóg loosely translated to "noble". Slavs were panentheistic and animistic, believing that everything was alive and was imbued with a distinct and separate spirit. Since trees, rocks, and animals were far older than humanity, their spirits were considered wiser, and were consulted for aid and advice. Slavs worshipped in groves and circles, rather than temples and they were accompanied by public offerings, feasting, and prophecy. They held firm beliefs in fairies and changelings, vampires and shape-shifters. They believed in reincarnation, stating that no new souls were ever born, and they honoured their ancestors, every home had a shrine to their own ancestors and twice yearly, at Spring and Zaduszki celebrated festivals honouring the dead. The soul they believed, existed separately from the body.

Nobility, however, came more into contact with outside peoples, and began to worship in the manner of these civilizations. It seems that the spirits that the peasants honoured were given temples and statues in their honour. This type of worship, was short-lived, having only just started a short time before Christianity swept through in the ninth and tenth centuries. In the fifteenth century, the great Polish writer Jan Dlugosz applied a great deal of zeal to rebuilding the Polish pantheon in the likeness of Greek and Roman Gods with the aid of a series of archaic expressions and old patronyms. Unfortunately, he seems to have drawn more from his own imagination rather than from mythological data. Tthis attempt among others that clouds any data claiming to prove that Polish Slavs had a pantheon of gods.

The embracing of Christianity by Polish Nobility came in the year of 966, when Poland officially became a state in the eyes of the West, and Mieszko ascended to the throne. Polish sister territory Lithuania, was the last pagan state in Europe, it became a Christian state when Duke Jagiello married into the Polish dynasty in 1374.