Objection 1. It would seem that it is lawful to refuse absolutely an appointment to the episcopate. For as Gregory says (Pastor. i, 7), "Isaias wishing to be of profit to his neighbor by means of the active life, desired the office of preaching, whereas Jeremias who was fain to hold fast to the love of his Creator by contemplation exclaimed against being sent to preach." Now no man sins by being unwilling to forgo better things in order to adhere to things that are not so good. Since then the love of God surpasses the love of our neighbor, and the contemplative life is preferable to the active, as shown above (25, 1; 26, 2;182, 1) it would seem that a man sins not if he refuse absolutely the episcopal office.
Objection 2. Further, as Gregory says (Pastor. i, 7), "it is very difficult for anyone to be able to know that he is cleansed: nor should anyone uncleansed approach the sacred ministry." Therefore if a man perceives that he is not cleansed, however urgently the episcopal office be enjoined him, he ought not to accept it.
Objection 3. Further, Jerome (Prologue, super Marc.) says that "it is related of the Blessed Mark* that after receiving the faith he cut off his thumb that he might be excluded from the priesthood." [This prologue was falsely ascribed to St. Jerome, and the passage quoted refers, not to St. Mark the Evangelist, but to a hermit of that name. (Cf. Baronius, Anno Christi, 45, num. XLIV)] Likewise some take a vow never to accept a bishopric. Now to place an obstacle to a thing amounts to the same as refusing it altogether. Therefore it would seem that one may, without sin, refuse the episcopal office absolutely.
On the contrary, Augustine says (Ep. xlviii ad Eudox.): "If Mother Church requires your service, neither accept with greedy conceit, nor refuse with fawning indolence"; and afterwards he adds: "Nor prefer your ease to the needs of the Church: for if no good men were willing to assist her in her labor, you would seek in vain how we could be born of her."
I answer that, Two things have to be considered in the acceptance of the episcopal office: first, what a man may fittingly desire according to his own will; secondly, what it behooves a man to do according to the will of another. As regards his own will it becomes a man to look chiefly to his own spiritual welfare, whereas that he look to the spiritual welfare of others becomes a man according to the appointment of another having authority, as stated above (1, ad 3). Hence just as it is a mark of an inordinate will that a man of his own choice incline to be appointed to the government of others, so too it indicates an inordinate will if a man definitively refuse the aforesaid office of government in direct opposition to the appointment of his superior: and this for two reasons.
First, because this is contrary to the love of our neighbor, for whose good a man should offer himself according as place and time demand: hence Augustine says (De Civ. Dei xix, 19) that "the demands of charity undertake an honest labor." Secondly, because this is contrary to humility, whereby a man submits to his superior's commands: hence Gregory says (Pastor. i, 6): "In God's sight humility is genuine when it does not obstinately refuse to submit to what is usefully prescribed."
Reply to Objection 1. Although simply and absolutely speaking the contemplative life is more excellent than the active, and the love of God better than the love of our neighbor, yet, on the other hand, the good of the many should be preferred to the good of the individual. Wherefore Augustine says in the passage quoted above: "Nor prefer your own ease to the needs of the Church," and all the more since it belongs to the love of God that a man undertake the pastoral care of Christ's sheep. Hence Augustine, commenting on John 21:17, "Feed My sheep," says (Tract. cxxiii in Joan.): "Be it the task of love to feed the Lord's flock, even as it was the mark of fear to deny the Shepherd."
Moreover prelates are not transferred to the active life, so as to forsake the contemplative; wherefore Augustine says (De Civ. Dei xix, 19) that "if the burden of the pastoral office be imposed, we must not abandon the delights of truth," which are derived from contemplation.
Reply to Objection 2. No one is bound to obey his superior by doing what is unlawful, as appears from what was said above concerning obedience (104, 5). Accordingly it may happen that he who is appointed to the office of prelate perceive something in himself on account of which it is unlawful for him to accept a prelacy. But this obstacle may sometimes be removed by the very person who is appointed to the pastoral cure--for instance, if he have a purpose to sin, he may abandon it--and for this reason he is not excused from being bound to obey definitely the superior who has appointed him. Sometimes, however, he is unable himself to remove the impediment that makes the pastoral office unlawful to him, yet the prelate who appoints him can do so--for instance, if he be irregular or excommunicate. On such a case he ought to make known his defect to the prelate who has appointed him; and if the latter be willing to remove the impediment, he is bound humbly to obey. Hence when Moses had said (Exodus 4:10): "I beseech thee, Lord, I am not eloquent from yesterday, and the day before," the Lord answered (Exodus 4:12): "I will be in thy mouth, and I will teach thee what thou shalt speak." At other times the impediment cannot be removed, neither by the person appointing nor by the one appointed--for instance, if an archbishop be unable to dispense from an irregularity; wherefore a subject, if irregular, would not be bound to obey him by accepting the episcopate or even sacred orders.
Reply to Objection 3. It is not in itself necessary for salvation to accept the episcopal office, but it becomes necessary by reason of the superior's command. Now one may lawfully place an obstacle to things thus necessary for salvation, before the command is given; else it would not be lawful to marry a second time, lest one should thus incur an impediment to the episcopate or holy orders. But this would not be lawful in things necessary for salvation. Hence the Blessed Mark did not act against a precept by cutting off his finger, although it is credible that he did this by the instigation of the Holy Ghost, without which it would be unlawful for anyone to lay hands on himself. If a man take a vow not to accept the bishop's office, and by this intend to bind himself not even to accept it in obedience to his superior prelate, his vow is unlawful; but if he intend to bind himself, so far as it lies with him, not to seek the episcopal office, nor to accept it except under urgent necessity, his vow is lawful, because he vows to do what it becomes a man to do.
Source: Summa Theologica (New Advent)