Albert Barnes on Titus 2:15

These things speak and exhort. 1Timm 6:2.

And rebuke with all authority. 1Timm 5:1, 1Timm 5:20; 2Ti 4:2. The word authority here means command επιταγη. 1Cor 7:6, 25, 2Cor 8:8, 1Timm 1:1, Tit 1:3. The sense here is, he was to do it decidedly, without ambiguity, without compromise, and without keeping any thing back. He was to state these things not as being advice or counsel, but as the requirement of God.

Let no man despise thee. That is, conduct yourself, as you may easily do, so as to command universal respect as a minister of God. 1Timm 4:12.

(l) "Let no man" 1Timm 4:12.