οὖσα looks like it refers to θρησκεία.Ἔστι δὲ καὶ ἡ μέχρι τοῦ νῦν κρατοῦσα λαοπλανὴς θρησκεία τῶν Ἰσμαηλιτῶν πρόδρομος οὖσα τοῦ ἀντιχρίστου. Κατάγεται δὲ ἀπὸ τοῦ Ἰσμαὴλ τοῦ ἐκ τῆς Ἄγαρ τεχθέντος τῷ Ἀβραάμ· διόπερ Ἀγαρηνοὶ καὶ Ἰσμαηλῖται προσαγορεύονται. Σαρακηνοὺς δὲ αὐτοὺς καλοῦσιν ὡς ἐκ τῆς Σάρρας κενοὺς διὰ τὸ εἰρῆσθαι ὑπὸ τῆς Ἄγαρ τῷ ἀγγέλῳ· Σάρρα κενήν με ἀπέλυσεν.
There is also the superstition of the Ishmaelites which to this day prevails and keeps people in error, being a forerunner of the Antichrist. They are descended from Ishmael, [who] was born to Abraham of Agar, and for this reason they are called both Agarenes and Ishmaelites. They are also called Saracens, which is derived from Sarras kenoi, or destitute of Sara, because of what Agar said to the angel: ‘Sara hath sent me away destitute.’
postscript: Reading some John of Damascus at the recommendation of Robert Spencer and others.