The Baptistère Saint-Jean is a Roman Catholic church in Poitiers, France. It is reputed to be the oldest existing Christian building in France, and is one of the most prominent examples of Merovingian architecture.
Wikipedia https://en.wikipedia.org/wiki/Baptistère_Saint-Jean
Address 50 Rue Saint-Simplicien, Poitiers 86000, France
Coordinates 46°34'46.154" N 0°20'54.901" E