Eustratios 19 was a monk, and later hegoumenos, of the monastery of the Agauroi (near Prousa, in Bithynia); he was a devoted and trusted follower of Ioannikios 2, who is alleged to have shown him special favour, foreseeing when Eustratios was still a monk that he would in due course become hegoumenos of the monastery (ᾔδει γὰρ τοῦτον εἰς ὕστερον τῆς ἀρχικῆς ἀξίας τοὺς οἴακας ἐμπιστευθησόμενον καὶ πατέρα πολλῶν προβάτων λογικῶν ἀναδειχθησόμενον); it was he who asked Petros 126 to write the Life of Ioannikios 2 and he was a major source of information: Petrus, Vita Ioannicii 12. He was one of three monks (the others were Sabas 10 and Theophylaktos 105) sent by the hegoumenos of the Agauroi monastery, Gregorios 48, to meet Ioannikios 2 when he returned from staying with Georgios 220; they built a cell (κελλίον) for him: Sabas, Vita Ioannicii 14.

According to the Life by Petrus, Eustratios 19 and Theophylaktos 105 were responsible for building cells for Ioannikios 2 on two occasions; the first was when he came back to the monastery in the early years after his withdrawal from the world, and Eustratios 19 and Theophylaktos 105 were sent by Gregorios 48 to prepare a cave for him to live in; the second was some fourteen years later, after he had lived in the cave for thirteen years and then spent some time seeking quietness in the Thrakesioi theme; on his return Eustratios 19 and Theophylaktos 105 were sent by Gregorios 48 to build a cell for him: Petrus, Vita Ioannicii 10-11.

In 811 Eustratios 19 was present when Ioannikios 2 foretold the death of the emperor Nikephoros 8 to relatives of the emperor: Petrus, Vita Ioannicii 14. Later the same year he visited Constantinople on business for his hegoumenos Gregorios 48 and carried back to Ioannikios 2 a query from the same persons about the fate of the emperor Staurakios 2: Petrus, Vita Ioannicii 15, Sabas, Vita Ioannicii 15.

Shortly after the accession of the emperor Leo V (Leo 15, in 813), Eustratios 19 was present when the emperor's nephew Bryennios 3 visited Ioannikios 2 with a query about the fate of Leo 15, and was told only that he would persecute the church: Petrus, Vita Ioannicii 17. Soon after the persecution was resumed under Leo 15, Eustratios 19 visited Ioannikios 2, who had retreated to Mt Alsos (near Lissos), to tell him about the persecution: Petrus, Vita Ioannicii 25. It was presumably at this time that he joined Ioannikios 2 at Lissos, leaving his monastery and remaining with him for a long period: Sabas, Vita Ioannicii 21 (αὐτῷ τε συνδιατρίψαι καὶ συναγωνίσασθαι μέχρι πολλοῦ). He later told Petros 126 about some miracles which Ioannikios 2 had performed while living near Lissos, how he killed a monster (ἔχιδνα) by the power of prayer and how he also killed a huge serpent by the power of prayer: Petrus, Vita Ioannicii 40, 46 (recounted as two separate incidents, but possibly two versions of the same incident). During the persecution Ioannikios 2 returned to Mt Olympus and hid there in a place known only to Eustratios 19: Petrus, Vita Ioannicii 30. This was on Mt Trichalix, near Prousa: Sabas, Vita Ioannicii 24.

In late 820 Eustratios 19 visited him and was told of the imminent death of the emperor Leo 15; he then left and stayed at the home of Niketas 154, where three days later the news arrived of the murder of Leo 15; he then returned to Ioannikios 2 to give him the news: Petrus, Vita Ioannicii 30, cf. Sabas, Vita Ioannicii 24 (he heard Ioannikios 2 predict the murder of Leo 15, and on his return to his monastery learned that Leo 15 had been murdered and Michael 10 was the new emperor). He and Niketas 154 went to see Ioannikios 2 to tell him of the accession of Michael 10: Sabas, Vita Ioannicii 25. They visited Ioannikos 2 for his blessing, and heard him prophesy the approaching death of the monk Elias 22; shortly afterwards Eustratios 19 was present when Elias 22 was taken ill and carried dying to the house of Leopardina 1: Petrus, Vita Ioannicii 31, cf. Sabas, Vita Ioannicii 25. When Theodoros 15 (Theodore the Stoudite) and other leading ecclesiastics came to see Ioannikios 2, it was Eustratios 19 who went to inform him of their arrival (for the date, possibly 821, cf. Theodoros 15): Sabas, Vita Ioannicii 28, Petrus, Vita Ioannicii 36.

He and his fellow monk Dositheos 2 were present when Isaakios 7 visited Ioannikios 2; as they were going to Constantinople on business (ἐπὶ τὴν διακονίαν) they returned with him and there tried to persuade his employer, the hegoumene of the women's monastery of Tou Kloubiou (Anonyma 56), to allow him to become a monk; she wavered under pressure from them but was finally hardened in her attitude by the backing of the monks of the Stoudite monastery (under Athanasios 8 and Naukratios 1; the date was therefore after the death of Theodoros 15, which was in 826); subsequently Eustratios was present when the hegoumene and her daughter, also a nun (Anonyma 57), visited Ioannikios 2; when Ioannikios 2 disconcerted the hegoumene by giving the staff of office to her daughter and not to her, Eustratios 19 explained to her, afterwards when they reached the suburb called Mandron, that Ioannikios 2 had foreseen that she was soon to die but would be succeeded as hegoumene by her daughter: Sabas, Vita Ioannicii 33, Petrus, Vita Ioannicii 57-58. He received the news from Ioannikios 2 that Inger 2, the iconoclast bishop of Nikaia, had just died, as he had recently foretold to Antonios 30 and Basilios 119: Petrus, Vita Ioannicii 38.

He accompanied Ioannikios 2 when the latter retreated from Mt Trichalix to the wilderness of Korakos Kephale, above the monastery of Antidios, to avoid the crowds attracted by his reputation: Sabas, Vita Ioannicii 34, Petrus, Vita Ioannicii 60. On the way he met travellers to whom Ioannikios 2 made himself invisible; to a query from them about his monastery (ποίας μονῆς) he said that he belonged to that of the Agauroi (τῶν Ἀγαύρων): Petrus, Vita Ioannicii 61. They reached the end of their journey in the heights above the monastery of Antidios, where they built cells for themselves: Petrus, Vita Ioannicii 62. Eustratios 19 later told Petros 126 about a number of miracles performed by Ioannikios 2 while living above the monastery of Antidios, which Petros 126 included in the Life: Petrus, Vita Ioannicii 62, cf. 62-67 (all probably late, under Theophilos 5; at least one occurred after the fall of Amorion in 838).

Eustratios 19 became hegoumenos of the monastery of the Agauroi after the deaths first of Gregorios 48 and then of Gregorios 48's nephew and successor, Anonymus 560: Sabas, Vita Ioannicii 32, Petrus, Vita Ioannicii 59 (καὶ τούτου τελευτήσαντος κατὰ τὴν ἀψευδῆ προφητείαν τοῦ ἀγγελομιμήτου πατρὸς ἡμῶν διαδέχεται τὴν ἡγεμονίαν ὁ πανοσιώτατος Εὐστράτιος, ὅστις καὶ μέχρι τῆς σήμερον, καθάπερ ἥλιος διαλάμπων, ἅπαντας τοὺς ὑπ' αὐτὸν ἰθυνομένους καταφωτίζει). He was hegoumenos during the iconoclast persecution under Theophilos 5, when he had to leave his monastery and flee from place to place; he was replaced by the iconoclast Antonios 31: Sabas, Vita Ioannicii 36 (συνέβη καὶ τὸν τῆς Ἀγαύρων μονῆς ποιμένα Εὐστράτιον ἐξελθόντα ἀπὸ τόπου εἰς τόπον διὰ Χριστὸν περιάγεσθαι). During the persecution he used to visit Ioannikios 2 frequently; on one occasion he arrived shortly after a visit paid to Ioannikios 2 by two of Theophilos 5's high officials, Anonymus 562 and Anonymus 563; he then told him about the cruelty of the persecutors, and Ioannikios 2 in reply foretold the Triumph of Orthodoxy after the death of Theophilos 5 and predicted that Methodios 1 would become patriarch of Constantinople: Sabas, Vita Ioannicii 46 ( τοῦ τιμίου ἀνδρὸς Εὐστρατίου, ἡγουμένου Ἀγαύρων), Petrus, Vita Ioannicii 69 (during the persecution Eustratios 19 often heard the saint make this prophecy). While hegoumenos he fell ill and sent the monk Nikolaos 71 to Ioannikios 2 to ask for his prayers; he made a partial recovery but later, while visiting one of his metochia, he fell ill again; he prayed to Ioannikios 2 and was rewarded by a vision of the saint, after which he recovered fully and returned to his monastery: Sabas, Vita Ioannicii 41 (τῷ οὖν πολλάκις μνημονευθέντι ὁσίῳ Εὐστρατίῳ, ἡγουμένῳ Ἀγαύρων), Petrus, Vita Ioannicii 54 (his vision of Ioannikios 2). He wrote an account of this episode himself, for the benefit of posterity: Petrus, Vita Ioannicii 54. While hegoumenos he once appealed to Ioannikios 2 for help and advice when a plague of caterpillars attacked the crops on the metochion of Sts Kosmas and Damian which belonged to the monastery of the Agauroi: Sabas, Vita Ioannicii 32, Petrus, Vita Ioannicii 56 (τῷ ἡγουμένῳ αὐτῶν, τῷ σεβασμίῳ λέγω Εὐστρατίῳ).

After Methodios 1 became patriarch and problems arose in the church, it was Eustratios 19 who sent to him the letter written by Ioannikios 2 (then in advanced old age), one result of which was the visit paid by Methodios 1 to Ioannikios 2, in November 846: Petrus, Vita Ioannicii 69. He was still alive when Petros 126 was writing the Life (see Petrus, Vita Ioannicii 59, cited above). He was a colleague of Ioannikios 2 and hegoumenos of the monastery of the Agauroi on Mt Olympus (τῷ αὐτῷ συναγωνιστῇ Εὐστρατίῳ τῷ θαυματουργῷ καὶ ἡγουμένῳ τῆς τῶν Ἀγαύρων μονῆς), when Theodora 2 restored the veneration of icons: Vita Mich. Sync. 26.

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