I think that Paul’s meaning is now sufficiently clear; though none of the commentators, so far as I perceive, have understood it. The general meaning is, that a selection ought to be made in admitting bishops, because it is a laborious and difficult charge; and that they who aim at it should carefully consider with themselves, whether or not they were able to bear so heavy a burden. Ignorance is always rash; and a mature knowledge of things makes a man modest. How comes it that they who have neither ability nor wisdom often aspire so confidently to hold the reins of government, but because they rush forward with their eyes shut? On this subject Quintilian remarked, that the ignorant speak boldly, while the greatest orators tremble.
For the purpose of restraining such rashness in desiring the office of a bishop, Paul states, first, that this is not an indolent rank, but a work; and next, that it is not any kind of work, but excellent, and therefore toilsome and full of difficulty, as it actually is. It is no light matter to be a representative of the Son of God, in discharging an office of such magnitude, the object of which is to erect and extend the kingdom of God, to procure the salvation of souls which the Lord himself hath purchased with his own blood, and to govern the Church, which is God’s inheritance. But it is not my intention at present to make a sermon, and Paul will again glance at this subject in the next chapter.
-from Calvin’s Commentary on 1 Timothy