πιστεύειν πιστοῦσθαι

From LSJ

οὗτος ἐγὼ ταχυτᾶτι· χεῖρες δὲ καὶ ἦτορ ἴσο → this is my speed: my hands and heart are its equal, such am I for speed; my hands and heart are just as good

Source

Greek > English (Woodhouse)

be certain, be positive