Besides Constantine the Great, the only other Byzantine emperor to be venerated by the Orthodox Church until the present day is the Nicene sovereign, John III Doukas Vatatzes. Considered by some a «father of the Greeks», Vatatzes has become an important part of modern Greek political discourse as well. Outlining the origin, evolution and diffusion of the sanctioned cult of John III as a saint-emperor, this paper suggests that the figure of St. Vatatzes has played an important role in shaping modern Greek identity, and aims to initiate further research on the topic so that the extraordinary endurance of his legacy will be better understood.