Adam Clarke on Titus 1:9

Holding fast the faithful word - Conscientiously retaining, and zealously maintaining, the true Christian doctrine, κατα την διδαχην, according to the instructions, or according to the institutions, form of sound doctrine, or confession of faith, which I have delivered to thee.

That he may be able by sound doctrine - If the doctrine be not sound, vain is the profession of it, and vain its influence. It is good to be zealously affected in a good thing; but zeal for what is not of God will do no good to the souls of men, how sincere soever that zeal may be.

To exhort - Them to hold the faith, that they may persevere.

And to convince - Refute the objections, confound the sophistry, and convert the gainsayers; and thus defend the truth.