Dionysius was Patriarch of Alexandria a.d. 247–265.  Basil’s “strange to say” is of a piece with the view of Dionysius’ heretical tendencies expressed in Letter ix. q.v.  Athanasius, however, (De Sent. Dionysii) was satisfied as to the orthodoxy of his predecessor.  Bp. Westcott (Dict. C. Biog. i. 851) quotes Lumper (Hist. Pat. xii. 86) as supposing that Basil’s charge against Dionysius of sowing the seeds of the Anomœan heresy was due to imperfect acquaintance with his writings.  In Letter clxxxviii. Basil calls him “the Great,” which implies general approval.