Cat. xiii. 4.  In the letter to Constantius, three or four years later than this Lecture, Cyril treats the appearance at that time of a luminous Cross in the sky as a fulfilment of Matt. xxiv. 30:  but he there adds (Ep. ad Constantium, § 6) that our Lord’s prediction “was both fulfilled at that present time, and shall again be fulfilled more largely.”  On the opinion that “the sign of the Son of Man in heaven” should be the Cross, see Suicer, Thesaurus, Σταυρός.  It is not improbable that the earliest trace of this interpretation is found in The Teaching of the Apostles, § 16:  “Then shall appear the signs of the Truth:  the first the sign of a (cross) spreading out (ἐκπετάσεως) in heaven.”