 In the early 1900s , Białystok was reputed to have the largest concentration of Jews of all the cities in the world . In 1931 , 40 @,@ 000 Jews lived in the city , nearly half the city 's inhabitants . The city is the seat of the Roman Catholic Archdiocese of Białystok . Pope John Paul II on 5 June 1991 , during a visit to Białystok , announced the establishment of the Archdiocese of Białystok which ended the period of the temporary church administration of the portion of the Archdiocese of Vilnius that had , after World War II , remained within the Polish borders . The city is also the seat of the Białystok @-@ Gdansk Diocese of the Autocephalous Polish Orthodox Church . Białystok is the largest concentration of Orthodox believers in Poland . In Białystok , the following Protestant churches exist : a Lutheran parish , two Pentecostal churches , Baptist church , a congregation of the Church of God in Christ and a Seventh Day Adventist church . 
