Athanasios 18 was a monk and hesychast; he was the addressee of letters from the patriarch Photius (Photios 1), written between 859 and 867, possibly between May 866 and Sept. 867: Photius, Ep. 20 (I 71f. Laourdas-Westerink) (addressed Ἀθανασίῳ μοναχῷ καὶ ἡσυχάζοντι), Ep. 26 (I 76 Laordas-Westerink) (addressed Ἀθανασίῳ μοναχῷ καὶ ἡσυχάζοντι), Ep. 27 (addressed τῷ αὐτῷ) (I 77 Laourdas-Westerink). He also received two other letters from Photius, of uncertain date: Photius, Ep. 93 (I 129f. Laourdas-Westerink) and 212 (II 115 Laourdas-Westerink) (both addressed Ἀθανασίῳ μονάζοντι καὶ ἡσυχαστῇ). He restored to the Church the schismatic bishop of Eriste (Anonymus 618); he also sent gifts to Photios 1, who in return sent him medicines: Photius, Ep. 20.

His recommendation of a person (Anonymus 615) to be hegoumenos of the Monastery of the Symboloi was approved by Photios 1, though it led to a revolt by the monks (Anonymi 20) who opposed the choice: Photius, Epp. 26, 27. One letter of his to Photios 1 was delivered by Anonymus 616, a monk from the monastery of the Symboloi: Photius, Ep. 27. Athanasios 18 had chosen the life of solitary contemplation for himself: Photius, Ep. 93. Photios 1 sent a cross to him as a gift: Photius, Ep. 212. Athanasios 18 is styled ἡ ὑμέτερα ὁσιότης or similar, Photius, Ep. 20, 26, 27; and ἡ ὑμέτερα ἁγιωσύνη, Ep. 27. He is addressed as πατέρ: Ep. 26. He evidently lived not far from the Monastery of the Symboloi, on Bithynian Olympus.

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