Diadochus, Marcus, a Greek theologian, who probably lived in the 4th century. Nothing is known of him except that he was the author of a small work against the Arians, entitled Τοῦ μακαρίου τοῦ Διαδόχου κατὰ Α᾿ρειανῶν λόγος (Beati Marci Diadochi Sernmo contra Arianos), and published by J. K. Wetstein as an appendix to his edition of Origen's De Oratione (Basel, 1694, 4to; reprinted, with a new Latin translation, in the Bibliotheca Patrum of Galland, vol. v). Some writers suppose that Diadochus was one of two Egyptian bishops by the name of Mark who were banished by the Arians during the patriarchate of George of Cappadocia, restored during the reign of Julian, and mentioned in a letter of Athanasius to the Antiochians as being present at the synod held at Alexandria in 362. According to the conjecture of Galland, Mark Diadochus was one of two bishops named Mark who were ordained priests by Alexander, the predecessor of Athanasius, and exiled by the Arians; the one to the great oasis (Upper Egypt), the other to the oasis of Ammon. It may be that these two Marks were the same as the two preceding ones. — Hoefer, Biographie Generale, 14:21.