The St. Willibrordus Church in the city centre of Utrecht (The Netherlands) is a prime example of the Gothic Revival architecture. The richly decorated church interior is, unfortunately, one of the best kept secrets of the city. In the semi-permanent art installation presented in this paper, imagery showing the church interior is projected on the outside walls of the entrance of the church. By projecting onto a spherical mirror suspended from the entrance ceiling, the projection is reflected onto three walls and the ceiling. The resulting image, when seen from the street-level outside, creates a perspective illusion. This paper shows how the installation was implemented in the church in an unobtrusive way, and explains the calibration system that was developed for determining the pre-distortions required to project the Trompe L'Oeil.