The three-aisled evangelical collegiate church of St. Marien dates from the 14th century and is believed to be on the foundations of two Romanesque predecessor buildings. Construction of the choir began in 1388, followed by the elevation of the parish church to a collegiate church in 1481. A library was set up on the upper floor of the late Gothic building, for which the scholar Konrad Wellin donated around 63 books as the basis in 1448. Today the collection comprises around 900 medieval manuscripts and early prints, including one of the three so-called "Wertheim Bibles" that still exist. The tombs of the Wertheim counts can be found on the ground floor of the choir.
In the course of the Reformation, the collegiate church gradually became a Protestant church from 1522. Around 100 years later, the count's family split into a Protestant and a re-Catholicized branch - from now on the collegiate church was used as a simultaneous church: Catholic masses were held in the choir, while Protestant services were celebrated in the nave. This simultaneous use of the church lasted until the completion of the Catholic parish church of St. Venantius in 1842. As a special feature, the collegiate church has a slightly asymmetrical floor plan, which is due on the one hand to the course of the street at that time and on the other hand to possible planning errors. Next to the entrance to the Wertheim collegiate church, a winding sandstone staircase leads to a small side room in the tower: the Heilig Geist Chapel. The Wertheim Madonna from the early 14th century hangs above the entrance to this chapel. It shows the change from the majestic medieval image of Mary to a loving mother.