The Cathedral of St John the Baptist, a large Baroque church at the edge of the old town, was built by the Jesuits in the 17th century and became a cathedral in 1805. The interior is more elaborate than the exterior suggests — trompe-l'oeil ceiling paintings give the nave a false dome effect, a technique common in Jesuit churches of the period. The acoustic whispering gallery beneath one of the chapels is worth finding.
The church holds the tomb of St Andrew Bobola, a Jesuit martyr who was killed by Cossacks in 1657 and later canonised. His relics were moved here in the 19th century and remain a pilgrimage site for Polish Catholics.