As with many works of unknown authorship and provenance, the significance of the Cohortatio ad Graeco,, once mistakenly included with the writings of Justin Martyr, is difficult to appreciate. Christoph Riedweg (1994) argued that Marcellus of Ancyra had authored the Cohortatio, a thesis that is now widely accepted (Arcari , Pouderon , Simonetti ). Although the focus on monotheism may fit Marcellus’ preoccupations in the context of the Council of Nicaea (Arcari, 2011), this perspective does not explain the author’s overarching apologetic aim, namely to convince “Hellenes” or pagans of the truth of monotheism against the way that they read their traditional canon of poets, philosophers and oracles. The author argues, not on the basis of Scripture, but through references in Greek literature and philosophy. Building on the work of Robert Grant (1958), I will argue that a careful comparison with Lactantius’ Divine Institutes show the Cohortatio to be responding to the same late Platonist theology that motivated the Latin author and surfaced in the period before the Great Persecution. The measured tone of the Cohortatio’s author suggests that Grant’s third century date may be correct. Nevertheless, the possibility that the treatise was written after the persecution remains.