Objection 1. It seems that a bishop cannot lawfully forsake his episcopal cure in order to enter religion. For no one can lawfully pass from a more perfect to a less perfect state; since this is "to look back," which is condemned by the words of our Lord (Luke 9:62), "No man putting his hand to the plough, and looking back, is fit for the kingdom of God." Now the episcopal state is more perfect than the religious, as shown above (Question 184, Article 07). Therefore just as it is unlawful to return to the world from the religious state, so is it unlawful to pass from the episcopal to the religious state.
Objection 2. Further, the order of grace is more congruous than the order of nature. Now according to nature a thing is not moved in contrary directions; thus if a stone be naturally moved downwards, it cannot naturally return upwards from below. But according to the order of grace it is lawful to pass from the religious to the episcopal state. Therefore it is not lawful to pass contrariwise from the episcopal to the religious state.
Objection 3. Further, in the works of grace nothing should be inoperative. Now when once a man is consecrated bishop he retains in perpetuity the spiritual power of giving orders and doing like things that pertain to the episcopal office: and this power would seemingly remain inoperative in one who gives up the episcopal cure. Therefore it would seem that a bishop may not forsake the episcopal cure and enter religion.
On the contrary, No man is compelled to do what is in itself unlawful. Now those who seek to resign their episcopal cure are compelled to resign (Extra, de Renunt. cap. Quidam). Therefore apparently it is not unlawful to give up the episcopal cure.
I answer that, The perfection of the episcopal state consists in this that for love of God a man binds himself to work for the salvation of his neighbor, wherefore he is bound to retain the pastoral cure so long as he is able to procure the spiritual welfare of the subjects entrusted to his care: a matter which he must not neglect--neither for the sake of the quiet of divine contemplation, since the Apostle, on account of the needs of his subjects, suffered patiently to be delayed even from the contemplation of the life to come, according to Philippians 1:22-25, "What I shall choose I know not, but I am straitened between two, having a desire to be dissolved, and to be with Christ, a thing by far better. But to abide still in the flesh is needful for you. And having this confidence, I know that I shall abide"; nor for the sake of avoiding any hardships or of acquiring any gain whatsoever, because as it is written (John 10:11), "the good shepherd giveth his life for his sheep."
At times, however, it happens in several ways that a bishop is hindered from procuring the spiritual welfare of his subjects. Sometimes on account of his own defect, either of conscience (for instance if he be guilty of murder or simony), or of body (for example if he be old or infirm), or of irregularity arising, for instance, from bigamy. Sometimes he is hindered through some defect in his subjects, whom he is unable to profit. Hence Gregory says (Dial. ii, 3): "The wicked must be borne patiently, when there are some good who can be succored, but when there is no profit at all for the good, it is sometimes useless to labor for the wicked. Wherefore the perfect when they find that they labor in vain are often minded to go elsewhere in order to labor with fruit." Sometimes again this hindrance arises on the part of others, as when scandal results from a certain person being in authority: for the Apostle says (1 Corinthians 8:13): "If meat scandalize my brother, I will never eat flesh": provided, however, the scandal is not caused by the wickedness of persons desirous of subverting the faith or the righteousness of the Church; because the pastoral cure is not to be laid aside on account of scandal of this kind, according to Matthew 15:14, "Let them alone," those namely who were scandalized at the truth of Christ's teaching, "they are blind, and leaders of the blind."
Nevertheless just as a man takes upon himself the charge of authority at the appointment of a higher superior, so too it behooves him to be subject to the latter's authority in laying aside the accepted charge for the reasons given above. Hence Innocent III says (Extra, de Renunt., cap. Nisi cum pridem): "Though thou hast wings wherewith thou art anxious to fly away into solitude, they are so tied by the bonds of authority, that thou art not free to fly without our permission." For the Pope alone can dispense from the perpetual vow, by which a man binds himself to the care of his subjects, when he took upon himself the episcopal office.
Reply to Objection 1. The perfection of religious and that of bishops are regarded from different standpoints. For it belongs to the perfection of a religious to occupy oneself in working out one's own salvation, whereas it belongs to the perfection of a bishop to occupy oneself in working for the salvation of others. Hence so long as a man can be useful to the salvation of his neighbor, he would be going back, if he wished to pass to the religious state, to busy himself only with his own salvation, since he has bound himself to work not only for his own but also for others' salvation. Wherefore Innocent III says in the Decretal quoted above that "it is more easily allowable for a monk to ascend to the episcopacy, than for a bishop to descend to the monastic life. If, however, he be unable to procure the salvation of others it is meet he should seek his own."
Reply to Objection 2. On account of no obstacle should a man forego the work of his own salvation, which pertains to the religious state. But there may be an obstacle to the procuring of another's salvation; wherefore a monk may be raised to the episcopal state wherein he is able also to work out his own salvation. And a bishop, if he be hindered from procuring the salvation of others, may enter the religious life, and may return to his bishopric should the obstacle cease, for instance by the correction of his subjects, cessation of the scandal, healing of his infirmity, removal of his ignorance by sufficient instruction. Again, if he owed his promotion to simony of which he was in ignorance, and resigning his episcopate entered the religious life, he can be reappointed to another bishopric [Cap. Post translat., de Renunt.]. On the other hand, if a man be deposed from the episcopal office for some sin, and confined in a monastery that he may do penance, he cannot be reappointed to a bishopric. Hence it is stated (VII, qu. i, can. Hoc nequaquam): "The holy synod orders that any man who has been degraded from the episcopal dignity to the monastic life and a place of repentance, should by no means rise again to the episcopate."
Reply to Objection 3. Even in natural things power remains inactive on account of a supervening obstacle, for instance the act of sight ceases through an affliction of the eye. So neither is it unreasonable if, through the occurrence of some obstacle from without, the episcopal power remain without the exercise of its act.
Source: Summa Theologica (New Advent)