It was long assumed that the remains of the Blessed Idesbald Van der Gracht date from the twelfth century. A thoroughgoing scientific investigation in 2015 has revealed that the bones are 300 years younger. This does not detract in any way from the devotional history as such, and will not diminish the devotion of the believers. All over the church you are reminded of the Blessed Idesbald: a Gothic shrine made to order by Jean de Bethune, stained-glass windows in the church recounting the events in his life, and a painting in the Gothic nave. Idesbald Van der Gracht was abbot of the Abbey of the Dunes in Koksijde. It was decided in 1239 to rebury his remains within the walls of the new monastery; when the coffin was opened, his body appeared to be intact. In 1623, at the height of the religious conflict between the Calvinists and the Catholic Spanish rulers, the coffin was exhumed once more to be taken to a safer place. It was opened again, and the body was still intact. This sparked off a real pilgrimage, fuelled by the miracle stories that surrounded his remains.
The visit by archduchess Isabella, which is depicted on the stained-glass windows in the choir, magnified the prestige of the relics and the abbey. In 1627, the lead coffin containing the body of Idesbald was moved to the new Abbey of the Dunes in Bruges, next to the convent of Our Lady of the Potterie. The building currently houses the Great Seminary. In 1831, Nicolaus de Roovere, the last monk of the Abbey of the Dunes, decided to donate the remains of Idesbald to the Augustinian nuns of Our Lady of the Potterie. It was not until 1894 that attempts to have the veneration of Idesbald Van der Gracht officially recognized by the ecclesiastical authority were rewarded with the beatification by Pope Leo XIII. The nuns of Our Lady of the Potterie did not wait for this decision; they already commissioned a neo-Gothic shrine that was finished in 1892 after a design by Jean de Bethune. In 1896, the neo-Gothic burial chapel was built to accommodate the shrine with the remains of Idesbald. Every year, on 18 April, the name day of Idesbald, the shrine was brought out of the burial chapel and placed in the middle of the church. The veneration of the Blessed Idesbald Van der Gracht brought many believers to Bruges to invoke him against rheumatic diseases such as arthritis.