But speak thou the things which become sound doctrine: But ... thou - in contrast to the reprobate seducers, Titus 1:11; Titus 1:15-16. 'He deals more in exhortations, because those intent on useless questions needed chiefly to be recalled to a holy life; for nothing so allays men's wandering curiosity as the being brought to recognize practical duties' (Calvin).
Speak - without restraint, with open mouth: contrast Titus 1:11, "mouths must be stopped."