Middle voice from martus; to be adduced as a witness, i.e. (figuratively) to obtest (in affirmation or exhortation) -- take to record, testify.
see GREEK martus
1. to cite a witness, bring forward a witness, call to witness (Tragg., Thucydides, Plato, and following); to affirm by appeal to God, to declare solemnly, protest: ταῦτα, Plato, Phil., p. 47 c.; ὅτι, Acts 20:26; Galatians 5:3.
2. to conjure, beseech as in God's name, exhort solemnly: τίνι, Acts 26:22 L T Tr WH; followed by the accusative with the infinitive, Ephesians 4:17; εἰς τό followed by accusative with an infinitive (cf. Buttmann, § 140, 10, 3), 1 Thessalonians 2:12-11T Tr WH. (Compare: διαμαρτύρομαι, προμαρτύρομαι.)