John Calvin Commentary Titus 2:10

"not purloining, but showing all good fidelity; that they may adorn the doctrine of God our Saviour in all things." — Titus 2:10 (ASV)

Not thievish but shewing all good faith. He censures two vices that are common among servants: petulant replies and a tendency to steal. The comedies are full of instances of glib talk, by which servants cheat their masters. Nor was it without reason that an exchange of names took place in ancient times, by which “servant” and “thief” became interchangeable terms. Thus, wisdom requires that we tailor our instructions to the character of each individual.

By faith he means faithfulness to their masters; and therefore, to shew all faith is to act faithfully, without using fraud or doing injury, in handling the affairs of their masters.

That they may adorn the doctrine of God our Savior in all things. This should be a powerful spur for us, when we learn that our appropriate conduct adorns the doctrine of God, which is also a mirror of his glory. And indeed, we see that this usually happens; just as, on the other hand, our wicked life brings disgrace upon it, for people commonly judge us by our actions. But this circumstance should also be noted: God condescends to receive an “ornament” from slaves, whose condition was so lowly and despised that they were usually scarcely considered men. For he does not mean “servants” like those we have today, but slaves, who were bought with money and held as property, like oxen or horses. And if the lives of these men are an ornament to the Christian name, how much more should those who are in positions of honor take care that they do not stain it by their wickedness.