Objection 1. It would seem that one who is appointed to the episcopate ought to be better than others. For our Lord, when about to commit the pastoral office to Peter, asked him if he loved Him more than the others. Now a man is the better through loving God the more. Therefore it would seem that one ought not to be appointed to the episcopal office except he be better than others.
Objection 2. Further, Pope Symmachus says (can. Vilissimus I, qu. 1): "A man is of very little worth who though excelling in dignity, excels not in knowledge and holiness." Now he who excels in knowledge and holiness is better. Therefore a man ought not to be appointed to the episcopate unless he be better than others.
Objection 3. Further, in every genus the lesser are governed by the greater, as corporeal things are governed by things spiritual, and the lower bodies by the higher, as Augustine says (De Trin. iii, 3). Now a bishop is appointed to govern others. Therefore he should be better than others.
On the contrary, The Decretal [Can. Cum dilectus, de Electione] says that "it suffices to choose a good man, nor is it necessary to choose the better man."
I answer that, In designating a man for the episcopal office, something has to be considered on the part of the person designate, and something on the part of the designator. For on the part of the designator, whether by election or by appointment, it is required that he choose such a one as will dispense the divine mysteries faithfully. These should be dispensed for the good of the Church, according to (1 Corinthians 14:12), "Seek to abound unto the edifying of the Church"; and the divine mysteries are not committed to men for their own meed, which they should await in the life to come. Consequently he who has to choose or appoint one for a bishop is not bound to take one who is best simply, i.e. according to charity, but one who is best for governing the Church, one namely who is able to instruct, defend, and govern the Church peacefully. Hence Jerome, commenting on Titus 1:5, says against certain persons that "some seek to erect as pillars of the Church, not those whom they know to be more useful to the Church, but those whom they love more, or those by whose obsequiousness they have been cajoled or undone, or for whom some person in authority has spoken, and, not to say worse than this, have succeeded by means of gifts in being made clerics."
Now this pertains to the respect of persons, which in such matters is a grave sin. Wherefore a gloss of Augustine [Ep. clxvii ad Hieron.] on (James 2:1), "Brethren, have not... with respect of persons," says: "If this distinction of sitting and standing be referred to ecclesiastical honors, we must not deem it a slight sin to 'have the faith of the Lord of glory with respect of persons.' For who would suffer a rich man to be chosen for the Church's seat of honor, in despite of a poor man who is better instructed and holier?"
On the part of the person appointed, it is not required that he esteem himself better than others, for this would be proud and presumptuous; but it suffices that he perceive nothing in himself which would make it unlawful for him to take up the office of prelate. Hence although Peter was asked by our Lord if he loved Him more than the others, he did not, in his reply, set himself before the others, but answered simply that he loved Christ.
Reply to Objection 1. Our Lord knew that, by His own bestowal, Peter was in other respects fitted to govern the Church: wherefore He questioned him about his greater love, to show that when we find a man otherwise fitted for the government of the Church, we must look chiefly to his pre-eminence in the love of God.
Reply to Objection 2. This statement refers to the pursuits of the man who is placed in authority. For he should aim at showing himself to be more excellent than others in both knowledge and holiness. Wherefore Gregory says (Pastor. ii, 1) "the occupations of a prelate ought to excel those of the people, as much as the shepherd's life excels that of his flock." But he is not to be blamed and looked upon as worthless if he excelled not before being raised to the prelacy.
Reply to Objection 3. According to (1 Corinthians 12:4) seqq., "there are diversities of graces... and... of ministries... and... of operations." Hence nothing hinders one from being more fitted for the office of governing, who does not excel in the grace of holiness. It is otherwise in the government of the natural order, where that which is higher in the natural order is for that very reason more fitted to dispose of those that are lower.
Source: Summa Theologica (New Advent)