τὸ γὰρ βραχύ τι τοῦτο πᾶσαν ὑμῶν ἔχει τὴν βεβαίωσιν καὶ πεῖραν τῆς γνώμης → this trifle contains the whole seal and trial of your resolution
(see also: ἑαυτοῦ) reflexive
