He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it.
Titus 1:9 (ESV)
"That he may be able. The pastor ought to have two voices one, for gathering the sheep ; and another, for warding off and driving away wolves and thieves. The Scripture supplies him with the means of doing both; for he who is deeply skilled in it will be able both to govern those who are teachable, and to refute the enemies of the truth. This twofold use of Scripture Paul describes when he says, That he may be able to exhort and to convince adversaries. And hence let us learn, first, what is the true knowledge of a bishop, and, next, to what purpose it ought to be applied. That bishop is truly wise, who holds the right faith; and he makes a proper use of his knowledge, when he applies it to the edification of the people.
This is remarkable applause bestowed on the word of God, when it is pronounced to be sufficient, not only for governing the teachable, but for subduing the obstinacy of enemies. And, indeed, the power of truth revealed by the Lord is such that it easily vanquishes all falsehoods. Let the Popish bishops now go and boast of being the successors of the apostles, seeing that the greater part of them are so ignorant of all doctrine, as to reckon ignorance to be no small part of their dignity."
John Calvin, Commentaries on the Epistles to Timothy, Titus, and Philemon.