Objection 1. It would seem that it is unlawful for a bishop, on account of some temporal persecution, to withdraw his bodily presence from the flock committed to his care. For our Lord said (John 10:12) that he is a hireling and no true shepherd, who "seeth the wolf coming, and leaveth the sheep and flieth": and Gregory says (Hom. xiv in Ev.) that "the wolf comes upon the sheep when any man by his injustice and robbery oppresses the faithful and the humble." Therefore if, on account of the persecution of a tyrant, a bishop withdraws his bodily presence from the flock entrusted to his care, it would seem that he is a hireling and not a shepherd.
Objection 2. Further, it is written (Proverbs 6:1): "My son, if thou be surety for thy friend, thou hast engaged fast thy hand to a stranger," and afterwards (Proverbs 6:3): "Run about, make haste, stir up thy friend." Gregory expounds these words and says (Pastor. iii, 4): "To be surety for a friend, is to vouch for his good conduct by engaging oneself to a stranger. And whoever is put forward as an example to the lives of others, is warned not only to watch but even to rouse his friend." Now he cannot do this if he withdraw his bodily presence from his flock. Therefore it would seem that a bishop should not on account of persecution withdraw his bodily presence from his flock.
Objection 3. Further, it belongs to the perfection of the bishop's state that he devote himself to the care of his neighbor. Now it is unlawful for one who has professed the state of perfection to forsake altogether the things that pertain to perfection. Therefore it would seem unlawful for a bishop to withdraw his bodily presence from the execution of his office, except perhaps for the purpose of devoting himself to works of perfection in a monastery.
On the contrary, our Lord commanded the apostles, whose successors bishops are (Matthew 10:23): "When they shall persecute you in this city, flee into another."
I answer that, In any obligation the chief thing to be considered is the end of the obligation. Now bishops bind themselves to fulfil the pastoral office for the sake of the salvation of their subjects. Consequently when the salvation of his subjects demands the personal presence of the pastor, the pastor should not withdraw his personal presence from his flock, neither for the sake of some temporal advantage, nor even on account of some impending danger to his person, since the good shepherd is bound to lay down his life for his sheep.
On the other hand, if the salvation of his subjects can be sufficiently provided for by another person in the absence of the pastor, it is lawful for the pastor to withdraw his bodily presence from his flock, either for the sake of some advantage to the Church, or on account of some danger to his person. Hence Augustine says (Ep. ccxxviii ad Honorat.): "Christ's servants may flee from one city to another, when one of them is specially sought out by persecutors: in order that the Church be not abandoned by others who are not so sought for. When, however, the same danger threatens all, those who stand in need of others must not be abandoned by those whom they need." For "if it is dangerous for the helmsman to leave the ship when the sea is calm, how much more so when it is stormy," as Pope Nicholas I says (cf. VII, qu. i, can. Sciscitaris).
Reply to Objection 1. To flee as a hireling is to prefer temporal advantage or one's bodily welfare to the spiritual welfare of one's neighbor. Hence Gregory says (Hom. xiv in Ev.): "A man cannot endanger himself for the sake of his sheep, if he uses his authority over them not through love of them but for the sake of earthly gain: wherefore he fears to stand in the way of danger lest he lose what he loves." But he who, in order to avoid danger, leaves the flock without endangering the flock, does not flee as a hireling.
Reply to Objection 2. If he who is surety for another be unable to fulfil his engagement, it suffices that he fulfil it through another. Hence if a superior is hindered from attending personally to the care of his subjects, he fulfils his obligation if he do so through another.
Reply to Objection 3. When a man is appointed to a bishopric, he embraces the state of perfection as regards one kind of perfection; and if he be hindered from the practice thereof, he is not bound to another kind of perfection, so as to be obliged to enter the religious state. Yet he is under the obligation of retaining the intention of devoting himself to his neighbor's salvation, should an opportunity offer, and necessity require it of him.
Source: Summa Theologica (New Advent)