Tit 2:1-15. Directions to Titus: How to Exhort Various Classes of Believers: The Grace of God in Christ Our Grand Incentive to Live Godly.
1. But … thou—in contrast to the reprobate seducers stigmatized in Tit 1:11, 15, 16. "He deals more in exhortations, because those intent on useless questions needed chiefly to be recalled to the study of a holy, moral life; for nothing so effectually allays men's wandering curiosity, as the being brought to recognize those duties in which they ought to exercise themselves" [Calvin].
speak—without restraint: contrast Tit 1:11, "mouths … stopped."
doctrine—"instruction" or "teaching."