Objection 1. It would seem unlawful for religious to occupy themselves with secular business. For in the decree quoted above (Article 1) of Pope Boniface it is said that the "Blessed Benedict bade them to be altogether free from secular business; and this is most explicitly prescribed by the apostolic doctrine and the teaching of all the Fathers, not only to religious, but also to all the canonical clergy," according to (2 Timothy 2:4), "No man being a soldier to God, entangleth himself with secular business." Now it is the duty of all religious to be soldiers of God. Therefore it is unlawful for them to occupy themselves with secular business.
Objection 2. Further, the Apostle says (1 Thessalonians 4:11): "That you use your endeavor to be quiet, and that you do your own business," which a gloss explains thus--"by refraining from other people's affairs, so as to be the better able to attend to the amendment of your own life." Now religious devote themselves in a special way to the amendment of their life. Therefore they should not occupy themselves with secular business.
Objection 3. Further, Jerome, commenting on Matthew 11:8, "Behold they that are clothed in soft garments are in the houses of kings," says: "Hence we gather that an austere life and severe preaching should avoid the palaces of kings and the mansions of the voluptuous." But the needs of secular business induce men to frequent the palaces of kings. Therefore it is unlawful for religious to occupy themselves with secular business.
On the contrary, The Apostle says (Romans 16:1): "I commend to you Phoebe our Sister," and further on (Romans 16:2), "that you assist her in whatsoever business she shall have need of you."
I answer that, As stated above (186, 1 and 7, ad 1), the religious state is directed to the attainment of the perfection of charity, consisting principally in the love of God and secondarily in the love of our neighbor. Consequently that which religious intend chiefly and for its own sake is to give themselves to God. Yet if their neighbor be in need, they should attend to his affairs out of charity, according to (Galatians 6:2), "Bear ye one another's burthens: and so you shall fulfil the law of Christ," since through serving their neighbor for God's sake, they are obedient to the divine love. Hence it is written (James 1:2)7): "Religion clean and undefiled before God and the Father, is this: to visit the fatherless and widows in their tribulation," which means, according to a gloss, to assist the helpless in their time of need.
We must conclude therefore that it is unlawful for either monks or clerics to carry on secular business from motives of avarice; but from motives of charity, and with their superior's permission, they may occupy themselves with due moderation in the administration and direction of secular business. Wherefore it is said in the Decretals (Dist. xxxviii, can. Decrevit): "The holy synod decrees that henceforth no cleric shall buy property or occupy himself with secular business, save with a view to the care of the fatherless, orphans, or widows, or when the bishop of the city commands him to take charge of the business connected with the Church." And the same applies to religious as to clerics, because they are both debarred from secular business on the same grounds, as stated above.
Reply to Objection 1. Monks are forbidden to occupy themselves with secular business from motives of avarice, but not from motives of charity.
Reply to Objection 2. To occupy oneself with secular business on account of another's need is not officiousness but charity.
Reply to Objection 3. To haunt the palaces of kings from motives of pleasure, glory, or avarice is not becoming to religious, but there is nothing unseemly in their visiting them from motives of piety. Hence it is written (2 Kings 4:13): "Hast thou any business, and wilt thou that I speak to the king or to the general of the army?" Likewise it becomes religious to go to the palaces of kings to rebuke and guide them, even as John the Baptist rebuked Herod, as related in Matthew 14:4.
Source: Summa Theologica (New Advent)