Library Theodoret Compendium of Heretical Fables
Book 2
On Marcellus
Marcellus the Galatian, much like this man, denied the triad of hypostases. He said that a certain extension of the Father’s deity came into Christ, and he called this God the Word; and that after the whole economy it was again drawn up and contracted toward the God from whom it had been stretched out. The all-holy Spirit he calls an extension of the extension, and says that this was granted to the apostles. And in short he posited a Triad that is stretched out and contracted according to different economies.
Greek: Theodoret of Cyrus, Haereticarum fabularum compendium / Αἱρετικῆς κακομυθίας ἐπιτομή (CPG 6223; PG 83, 336–556). English: study translation for BWAB Library—not a modern copyrighted translation. The full five books are hosted here.