Dictionary of Christian Biography and Literature to the End of the Sixth Century/Isaacus Senior, disciple of Ephraim the Syrian
Isaacus (29) Senior, mentioned in an anonymous Life of Ephraim the Syrian among the more distinguished disciples of Ephraim who were also Syriac writers. He is cited by Joannes Maro (Tract. ad Nest. et Eutych.), by Bar-hebraeus (Hist. Dynast. 91), and by many other Syriac and Arabic authors, most of whom, however, confuse him with Isaac presbyter of Antioch (Assemani, B. O. i. 165). Gennadius in his de Scriptor. Eccl. c. 26, says: "Isaac wrote, concerning the Three Persons of the Holy Trinity and the Incarnation of the Lord, a book of very dark disputation and involved discourse; proving that there are three Persons in the one Godhead, each possessing a proprium peculiar to himself. The proprium of the Father is that He is the origin of the others, yet Himself without origin; that of the Son is that, though begotten, He is not later than His begetter; that of the Holy Ghost is that It is neither made nor begotten, and yet is from another. Of the Incarnation he writes that two Natures abide in the one Person of the Son of God." This chapter precedes those about Marcarius and Evagrius Pontinus, who lived ante 400. It is hence inferred that Isaac flourished about the end of the 4th cent. (Cave, i. 415, places him c. 430 (?), but some put him a century earlier.)
The work of Isaac, not unfairly described by Gennadius, is entitled Libellus Fidei SS. Trinitatis et Incarnationis Domini. It is a brief treatise, and is printed in Migne, Patr. Gk. xxxiii. In a codex Pithoeanus, teste Sirmond, the title is Fides Isaacis (or Isacis) ex Judaeo. Hence Isaac Senior has been identified by Tillemont (viii. 409) with Isaac the converted Jew who calumniated pope Damasus. Assemani thinks that the silence of Gennadius and his epitomizer Honorius renders it doubtful that Isaac Senior, the author of the Libellus Fidei, was a Jew. Cf. also Galland. vii. Prol. p. xxv.; Ceillier, vi. 290; Mansi, iii. 504 B; Pagi, Crit. ad ann. 378, xx.