Objection 1. It would seem unlawful for religious to teach, preach, and the like. For it is said (VII, qu. i, can. Hoc nequaquam) in an ordinance of a synod of Constantinople [Pseudosynod held by Photius in the year 879]: "The monastic life is one of subjection and discipleship, not of teaching, authority, or pastoral care." And Jerome says (ad Ripar. et Desider. [Contra Vigilant. xvi]): "A monk's duty is not to teach but to lament." Again Pope Leo [Leo I, Ep. cxx ad Theodoret., 6, cf. XVI, qu. i, can. Adjicimus]: says "Let none dare to preach save the priests of the Lord, be he monk or layman, and no matter what knowledge he may boast of having." Now it is not lawful to exceed the bounds of one's office or transgress the ordinance of the Church. Therefore seemingly it is unlawful for religious to teach, preach, and the like.
Objection 2. Further, in an ordinance of the Council of Nicea (cf. XVI, qu. i, can. Placuit) it is laid down as follows: "It is our absolute and peremptory command addressed to all that monks shall not hear confessions except of one another, as is right, that they shall not bury the dead except those dwelling with them in the monastery, or if by chance a brother happen to die while on a visit." But just as the above belong to the duty of clerics, so also do preaching and teaching. Therefore since "the business of a monk differs from that of a cleric," as Jerome says (Ep. xiv ad Heliod.), it would seem unlawful for religious to preach, teach, and the like.
Objection 3. Further, Gregory says (Regist. v, Ep. 1): "No man can fulfil ecclesiastical duties, and keep consistently to the monastic rule": and this is quoted XVI, qu. i, can. Nemo potest. Now monks are bound to keep consistently to the monastic rule. Therefore it would seem that they cannot fulfil ecclesiastical duties, whereof teaching and preaching are a part. Therefore seemingly it is unlawful for them to preach, teach, and do similar things.
On the contrary, Gregory is quoted (XVI, qu. i, can. Ex auctoritate) as saying: "By authority of this decree framed in virtue of our apostolic power and the duty of our office, be it lawful to monk priests who are configured to the apostles, to preach, baptize, give communion, pray for sinners, impose penance, and absolve from sin."
I answer that, A thing is declared to be unlawful to a person in two ways. First, because there is something in him contrary to that which is declared unlawful to him: thus to no man is it lawful to sin, because each man has in himself reason and an obligation to God's law, to which things sin is contrary. And in this way it is said to be unlawful for a person to preach, teach, or do like things, because there is in him something incompatible with these things, either by reason of a precept--thus those who are irregular by ordinance of the Church may not be raised to the sacred orders--or by reason of sin, according to Psalm 49:16, "But to the sinner God hath said: Why dost thou declare My justice?"
In this way it is not unlawful for religious to preach, teach, and do like things, both because they are bound neither by vow nor by precept of their rule to abstain from these things, and because they are not rendered less apt for these things by any sin committed, but on the contrary they are the more apt through having taken upon themselves the practice of holiness. For it is foolish to say that a man is rendered less fit for spiritual duties through advancing himself in holiness; and consequently it is foolish to declare that the religious state is an obstacle to the fulfilment of such like duties. This error is rejected by Pope Boniface [Boniface IV] for the reasons given above. His words which are quoted (XVI, qu. i, can. Sunt. nonnulli) are these: "There are some who without any dogmatic proof, and with extreme daring, inspired with a zeal rather of bitterness than of love, assert that monks though they be dead to the world and live to God, are unworthy of the power of the priestly office, and that they cannot confer penance, nor christen, nor absolve in virtue of the power divinely bestowed on them in the priestly office. But they are altogether wrong." He proves this first because it is not contrary to the rule; thus he continues: "For neither did the Blessed Benedict the saintly teacher of monks forbid this in any way," nor is it forbidden in other rules. Secondly, he refutes the above error from the usefulness of the monks, when he adds at the end of the same chapter: "The more perfect a man is, the more effective is he in these, namely in spiritual works."
Secondly, a thing is said to be unlawful for a man, not on account of there being in him something contrary thereto, but because he lacks that which enables him to do it: thus it is unlawful for a deacon to say mass, because he is not in priestly orders; and it is unlawful for a priest to deliver judgment because he lacks the episcopal authority. Here, however, a distinction must be made. Because those things which are a matter of an order, cannot be deputed to one who has not the order, whereas matters of jurisdiction can be deputed to those who have not ordinary jurisdiction: thus the delivery of a judgment is deputed by the bishop to a simple priest. On this sense it is said to be unlawful for monks and other religious to preach, teach, and so forth, because the religious state does not give them the power to do these things. They can, however, do them if they receive orders, or ordinary jurisdiction, or if matters of jurisdiction be delegated to them.
Reply to Objection 1. It results from the words quoted that the fact of their being monks does not give monks the power to do these things, yet it does not involve in them anything contrary to the performance of these acts.
Reply to Objection 2. Again, this ordinance of the Council of Nicea forbids monks to claim the power of exercising those acts on the ground of their being monks, but it does not forbid those acts being delegated to them.
Reply to Objection 3. These two things are incompatible, namely, the ordinary cure of ecclesiastical duties, and the observance of the monastic rule in a monastery. But this does not prevent monks and other religious from being sometimes occupied with ecclesiastical duties through being deputed thereto by superiors having ordinary cure; especially members of religious orders that are especially instituted for that purpose, as we shall say further on (188, 4).
Source: Summa Theologica (New Advent)