Paganism competed with Christianity on Belarusian lands for quite a long time. Lithuanian tribes that had polytheism with gods similar to East-Slavic ones until the 15th century strengthened its positions. As recently as in the beginning of the 20th century, there was a pagan temple in Minsk near Lodačnaja street. A giant stone that was worshipped there is now part of Uručča Boulder Museum exposition.
One can still trace pagan heritage in Belarusian folklore and festive traditions. Check paganstva.org to find out more about paganism and its modern supporters in Belarus.