The crisis of the third century helped to create conditions for a change in terms of relations between the Roman Empire and Christianity, whose spread resulted, in the upper echelons of the State, in an awareness of the extent of the phenomenon. The measures that Roman State authorities took against Christianity were a reaction to this dangerous threat; among them, in the specific case of this paper, we focus on the persecution triggered – according to Lactantius – by the Caesar Galerius under the senior Augustus Diocletian. The same Galerius as Augustus was responsible for the formal act that put an end to persecutions: the edict of Serdica (or of Nicomedia), issued on 30 April of year 311. Apart from hypothetical personal reasons, the turning point is to be read in the context of traditional imperial clementia. As a result, Galerius emerges as an autonomous and leading figure within tetrarchical legislative policy, in the light of objective circumstances that led him to adapt Diocletian’s project to the needs of the times.