© 2018 by the authors. Heritage issues have increased significantly in recent years. However, they tend to remain in the cultural sphere and are often resistant to scientific analyses. If we have to deal with the contradictory matter of sustainability in design for ancient buildings, such hindrances appear frequently. A crucial aspect in Architecture has always been its capacity to dispose internal spaces and apertures in a manner that enhances the balance of light and thus provides attuned perception and well-being. Poor performance in that respect raised objections against the prestige of admirable works and famed artists. If we reject the absurd idea of accurately reproducing identical buildings in the same place repeatedly, how are we supposed to benefit from the said knowledge without the help of any objective design tools? It is easy to agree that at least we would need some scientific support to transmit such proper effects. Aware of the former notions, authors have developed a novel simulation software called DianaX, which is based on mathematical models and equations produced and expanded by Joseph Cabeza-Laïnez, from roughly 1990 to 2018. This non-commercial software deals with radiative exchanges in all kinds of surfaces (for instance domes, vaults, cylinders, hyperboloids and curves in general). It also includes direct sun in the simulations unlike most programs. Therefore, it is ideally suited for the analysis of heritage architecture and especially that which identifies with the Renaissance, baroque and neoclassical epochs. The case of temples from the baroque period resumes the conflict expressed in the first paragraph and the Jesuit Church of Saint Louis (1699-1731) is one of the most relevant examples of efficient illumination found in Mediterranean latitudes, having been recently restored. In this article, we would like to discuss the subtle and interesting implications of employing our simulation software for lighting in such a complex baroque temple. The methodology would be to identify the main energy sources within the church in order to construct a suitable model for simulation. Subsequently we apply the said software DianaX to such model and establish the most significant results trying to compare them with available on-site measurements. Finally, a strategy to enhance day-lighting and supplement it with other light sources in the church is proposed.