Abstract Among the works ascribed to Gregory Thaumaturgus, Gregory of Nazianzus, and Gregory of Nyssa there is one entitled, in Greek, Ad Evagrium monachum, de deitate, and, in Syriac, Ad Philagrium, de consubstantiali, which deals with the issue of whether the nature of Father, Son, and Holy Spirit is simple or composite. While the attributions to the Cappadocians have been ruled out on the basis of its Monarchian contents, current scholarship is still completely divided in dating it to the third or to the fourth century, so that the mutually exclusive hypotheses that the Thaumaturgus was its author and that Evagrius of Pontus was its addressee have continued to coexist. This study accounts for the investigations of previous scholars and focuses on those doctrinal contents which are attacked and endorsed in the work and which allow us to date it with some degree of certainty. It is argued that the main polemical focus of the Ad Evagrium was Eunomius’ theory of names and that it was written by an exponent of the Marcellian party in the late 370 s.