Built as the church of a powerful convent that became the centre of a small sovereign state, this church has been changed from a Romanesque church to an almost completely Gothic church, and became a simple parish church after the Stift was disbanded. It is the only surviving building of the abbey's complex.
Both outside and inside the building gives an indication of the former wealth of Thorn. Of the original Romanesque church, built in the second half of the 12th century, only the lower side of the tower and the crypt remain. The rest of the church was rebuilt in Gothic style in several stages, from the 13th until the 16th century. Despite this long period, the church is remarkably consistent in style and is a highlight of the regional Maasland Gothic style (see Gothicism). The interior however is mostly Baroque.
Three generations of the Cuypers family of architects worked on restorations to this church.
A restoration by P.J.H. Cuypers from 1860 until 1880 among others resulted in the completion of the original Romanesque tower with a new, neo-Gothic upper part made of brick and marl. This tower is in a sort of neo-Gothicism that resembles the Gothic style of the Duchy of Brabant more than that it has anything to do with this part of the country. An addition more in the style of the region are the traceries at the gables of transept and side-aisles at the north-side of the church. Also, the westwork of the church, which was in the typical fortress-like style of the Maas valley (compare it with the westwork of the Church of Our Lady in Maastricht) was spoiled by adding a window to it. In 1912 Jos. Cuypers reconstructed the westwork's southern stair-turret, as well as restoring the interior. The neo-Gothic upper part of the tower was reconstructed according to the original plans by Pierre Cuypers jr. in 1956-1957 after having been destroyed at the end of World War Two.