Cassianus (2) Julius a heretical teacher who lived towards the end of the 2nd cent. chiefly known to us by references to his writings made on two occasions by Clemens Alexandrinus. In the first passage (Strom. i. 21 copied by Eusebius Praep. Ev. x. 12) Clement engages in a chronological inquiry to shew the greatly superior antiquity of Moses to the founders of Grecian philosophy and he acknowledges himself indebted to the previous investigations made by Tatian in his work addressed to the Greeks and by Cassian (spelt Casianus in the MS. of Clement but not in those of Eusebius) in the first book of his Exegetica. Vallarsi (ii. 865) alters without comment the Cassianus of previous editors into Casianus in Jerome's Catalogue 33 a place where Jerome is not using Clement directly but is copying the notice in Eusebius (H. E. vi. 13). Jerome adds that he had not himself met the chronological work in question. In the second passage (Strom. iii. 13 seq.) Cassian is also named in connexion with Tatian. Clement is in this section refuting the doctrines of those Gnostics who in their view of the essential evil of matter condemned matrimony and the procreation of children; and after considering some arguments urged by Tatian says that similar ones had been used by Julius Cassianus whom he describes as the originator of Docetism (ὁ τῆς δοκήσεως ἐξάρχων) a statement which must be received with some modification.

Another specimen of Cassian's arguments in this treatise is preserved in Jerome's Commentary on Gal_6:8. Jerome there answers an Encratite argument founded on this text viz. that he who is united to a woman soweth to the flesh and therefore shall of the flesh reap corruption. This argument is introduced with words which according to the common reading run "Tatianus qui putativam Christi carnem introducens omnem conjunctionem masculi ad foeminam immundam arbitratur tali adversum nos sub occasione praesentis testimonii usus est argumento." There is little doubt that we are to read instead of Tatianus Cassianus. The Benedictine editor who retains the old reading notes that Cassianus is the reading of two of the oldest MSS. while Vallarsi says that Cassianus was the reading of every MS. he had seen.

The Docetism of Cassian was closely connected with his Encratism, for it was an obvious answer of the orthodox to his doctrine on Continence, that if the birth of children were essentially evil, then our Lord's own birth was evil, and His mother an object of blame. This was met by a denial of the reality of our Lord's body. Cassian also taught that man had not been originally created with a body like ours, but that these fleshly bodies were the "coats of skin" in which the Lord clothed our first parents after the Fall. This notion, probably derived from Valentinus (Iren. I. v. p. 27), had considerable currency. References for it will be found in Huet's Origeniana, ii. Qu. 12, viii., and Beausobre, Manichéisme, ii. 135).

Theodoret (Haer. Fab. i. 8) enumerates among the followers of Valentinus one Cossian, by whom, no doubt, Julius Cassianus is intended; for many greater inaccuracies in the names are in the present text of Theodoret, and Theodoret would have found authority in Clement for classing Cassian with Valentinus.

The coincidences between Tatian and Cassianus seem too close to be accidental, but we have not data to determine their relative priority. If Cassian were really the founder of the sect called Docetae, he must have been some time antecedent to Serapion (Eus. H. E. vi. 12). His country may have been Egypt (cf. Harnack, Gesch. der Alt. Chr. Lit. pp. 201–204). [See Docetae; Encratites.]