II. In no case ought bishops to be ordained hastily.
But if in every grade of the Church great forethought and knowledge has to be employed, lest there be any thing disorderly or out of place in the house of the Lord: how much more carefully must we strive to prevent mistakes in the election of him who is set over all the grades? For the peace and order of the Lord's whole household will be shaken, if what is required in the body be not found in the head. Where is that precept of the blessed Apostle Paul uttered through the Spirit of God, whereby in the person of Timothy the whole number of Christ's priests are instructed, and to each one of us is said: “Lay hands hastily on no one, and do not share in other men's sins?”
What is to lay on hands hastily but to confer the priestly dignity on unproved men before the proper age, before there has been time to test them, before they have deserved it by their obedience, before they have been tried by discipline? And what is to share in other men's sins but for the ordainer to become such as is he who ought not to have been ordained by him? For just as a man stores up for himself the fruit of his good work, if he maintains a right judgment in choosing a priest: so one who receives an unworthy priest into the number of his colleagues, inflicts grievous loss upon himself.
We must not then pass over in the case of any one that which is laid down in the general ordinances: nor is that advancement to be reckoned lawful which has been made contrary to the precepts of God's law.
Source: Letters (New Advent)