The Holy Mary church was built between 1253 and 1288 in iron sandstone holding limonite, and is early gothic style. The building has a three aisled nave and an encorbellant transept. The choir ends with a simple lancet arch, and the total construction is overvaulted with diagonal rib vault. The sober westfacade with its buttresses is broken with a sandstone rose window with tracery.
Before the construction of the actual church there stood a chapel, where the inhabitants of the castle, situated on the Warandeheuvel, could hear the mass. The oldest mention about the chapel is from 1211. The lord of Diest was responsible for the living sustainance of the chaplain appointed by the abbaye of Tongerlo . In 1233 Arnold IV, the lord of then of Diest, donated the chapel with its matching properties and tenths to the abbaye of Tongerlo. His condition was that the premonstransers of Tongerlo should look for the services.
The town grows fast, what makess that the chapel is too small. The first july 1253 the chapel is elevated to parish church.Because the room of the chapel does not satisfy, the construction of a larger church is started. It is inaugurated in march 1255 and in february 1`288 the construction of the church is completed.
The church is very damaged in 1580. The churchmasters refuse to pay a kind of ransom to the protestants, which then take all the metal out of the church. Most of the altars and artpieces are destroyed. To crown it all the tower collapses,with the crushing of the roof and vault as consequence. Only the walls still stand after this disaster.
The damage was restaured around 1600 and in 1608 the towerclock, that was hidden in the Liefkintgodshuis during the troubles, was replaced in the tower. Four years later the complete interior was rewhitened. Mechelen let melt a new clock around that time. The triumph cross standing above the choirentrance, is restorated in 1627. New glasswindows can be placed five years later thanks to gifts of the inhabitants of Diest.
A new belgian bluestone baroqueporche was added in 1777, in replacement for the gothic porche.
In the night of 2 to 3 july 1830 there was again a disaster. The cross rib vaulting of the choir collapses, and with this also some glasswindows,and also the pulpit from 1641. The colllaps from 1830 causes a complete and expensive renovation. Because of lack of money there are talks about letting the church go bakc to the nature. The parishpeople should go to the Beguinenchurch. But finaly the restaurationworks can be started according the plans of the architect P. Langerock in 1898-1899. He planned also the construction of a monumental westtower of 65 meter highth. But because of lack of money this would never be realized.
For more info (flemish): look at PatrimoniumDiest Bruidstraat