The former altarpiece dating back to 1596 and depicting the Assumption of Mary by Jan De Haen clearly demonstrates that Sint-Sulpitiuskerk is dedicated to the devotion of Mary. It was commissioned for the church’s new main altar because the previous one was damaged during the occupation of the city by William of Orange's troops between 1580 and 1583. It remained the altarpiece until the current main altar was erected in the 18th century. Today, the painting hangs to the left of the statue of Our Lady of Diest in the north transept. Jan de Haen was active as a painter in Diest during the last quarter of the 16th and early 17th centuries.
In the foreground of the painting Mary Magdalene and Mary Salome are standing near Mary's tomb. Behind them the apostles are looking in admiration at Mary being carried up to heaven by angels. Two angels are placing a crown on her head, symbolizing Mary as Queen of Heaven. Appearing between the clouds from left to right are Aaron, Moses with the stone tablets, Eve, David with his harp, Isaias and Adam. The scene is set against a background featuring a beautiful mountain landscape.