Theophilos 12 was bishop of Thessalonike; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and is attested at the fourth and seventh sessions only, on 1 October and 13 October 787; he probably attended the last five sessions, from 1 October to 23 November 787: Mansi XIII 133-486. In the list of those present at the seventh session he is styled Θεοφίλου ἐπισκόπου Θεσσαλονίκης: Mansi XIII 365. Elsewhere he is styled Θεόφιλος ἀνάξιος ἐπίσκοπος Θεσσαλονίκης: Mansi XIII 133. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 133. His name is missing from the list of those who subscribed the statement of the faith adopted by the Council at the seventh session; he attended this session and the omission is presumably a textual mistake.

Named in a list of archbishops of Thessalonike in the Synodikon of Orthodoxy: Gouillard, "Synodikon", p. 114, IX, line 5. Cf. also Petros 3 and Anastasios 3.

Possibly identical with the unnamed archbishop of Thessalonike in 797 when Theodoros 15 (Theodore the Stoudite) was exiled there: Theod. Stud., Epp. 1, 3. This unnamed archbishop could have been Thomas 4.

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