Fernand Pouillon (1912-1986) was one of the foremost European architects of the second half of the 20th century. A key figure in France's post-World War II reconstruction, Pouillon was responsible for a multitude of projects, including three significant residential building complexes on the outskirts of Algiers, Algeria, commissioned as part of the housing program promoted by Mayor Jacques Chevallier. These complexes are: Diar Es-Saâda (1953), Diar el Mahçoul (1954), and the most famous, Climat de France (1955-1959). Diar el Mahçoul was divided into two quarters: one for the European community, where Pouillon designed a church dedicated to Saint John the Baptist, and the other for the indigenous Arab population. A few years later, Pouillon was once again commissioned by the Algerian government to convert the church into a mosque, which was later named the Bachir Ibrahimi Mosque. This transformation included the addition of a striking minaret adorned with blue ceramics and embellished with pure gold. This paper seeks to explore the history behind this significant transition, delving into the architectural, typological, spatial, formal, and symbolic aspects of the mosque's conversion. For instance, the conversion involved addressing substantial differences between the orientation of a Christian church and that of a mosque, as well as considerations regarding the theme of light and the fundamental issue of decoration. The history of architecture is rich with examples of buildings adapted for new purposes, and Pouillon's work fits into this millennia-old tradition that recognizes the intrinsic value of architecture beyond its initial use, preserving it through its transition to a "second life". The Bachir Ibrahimi Mosque, now abandoned and vandalized, remains a relatively unknown building. This research aims to bring it back to the attention of both the public and professionals by retrieving archival material from Les Pierres Sauvages de Belcastel association and reconstructing its original image, which has unfortunately faded over time.