The central fact of Belarusian religious history is a church that no longer exists. The Union of Brest in 1596 brought most of the Orthodox hierarchy of the Grand Duchy of Lithuania into communion with Rome while keeping the Byzantine rite, the Church Slavonic liturgy and a married clergy. Over the following two centuries the Uniate church became the church of the Belarusian peasantry, and by some accounts held a substantial majority of the population of the territory. It was also, uniquely, an institution that was neither Polish Catholic nor Russian Orthodox, and that used the local language in preaching.
It was abolished by imperial decree at the Synod of Polotsk in 1839, its parishes absorbed into the Russian Orthodox Church and its books withdrawn. Whatever one thinks of the union itself, the abolition removed the one large religious institution that was specifically Belarusian, and the absence shapes the country still.
What remains is an Orthodox majority served by an exarchate of the Moscow Patriarchate, a Roman Catholic minority of roughly a tenth concentrated around Grodno and the Lithuanian border, and a small revived Greek Catholic community. Soviet antireligious policy was applied here with particular severity, and Belarus is among the least observant countries in the region.
There is also a Jewish absence. Belarus was a heartland of Ashkenazi life, of the Litvak yeshiva tradition centred on nearby Vilna and of Hasidic courts including Lubavitch, named for a Belarusian town. The Holocaust ended that, and the postwar Soviet state discouraged any commemoration that identified the victims as Jews.