Objection 1. It seems that it is lawful to give and receive money for spiritual actions. The use of prophecy is a spiritual action. But something used to be given of old for the use of prophecy, as appears from 1 Samuel 9:7-8, and 1 Kings 14:3. Therefore it would seem that it is lawful to give and receive money for a spiritual action.
Objection 2. Further, prayer, preaching, divine praise, are most spiritual actions. Now money is given to holy persons in order to obtain the assistance of their prayers, according to Luke 16:9, "Make unto you friends of the mammon of iniquity." To preachers also, who sow spiritual things, temporal things are due according to the Apostle (1 Corinthians 9:14). Moreover, something is given to those who celebrate the divine praises in the ecclesiastical office, and make processions: and sometimes an annual income is assigned to them. Therefore it is lawful to receive something for spiritual actions.
Objection 3. Further, science is no less spiritual than power. Now it is lawful to receive money for the use of science: thus a lawyer may sell his just advocacy, a physician his advice for health, and a master the exercise of his teaching. Therefore in like manner it would seem lawful for a prelate to receive something for the use of his spiritual power, for instance, for correction, dispensation, and so forth.
Objection 4. Further, religion is the state of spiritual perfection. Now in certain monasteries something is demanded from those who are received there. Therefore it is lawful to demand something for spiritual things.
On the contrary, It is stated (I, qu. i [Can. Quidquid invisibilis]): "It is absolutely forbidden to make a charge for what is acquired by the consolation of invisible grace, whether by demanding a price or by seeking any kind of return whatever." Now all these spiritual things are acquired through an invisible grace. Therefore it is not lawful to charge a price or return for them.
I answer that, Just as the sacraments are called spiritual, because they confer a spiritual grace, so, too, certain other things are called spiritual, because they flow from spiritual grace and dispose thereto. And yet these things are obtainable through the ministry of men, according to 1 Corinthians 9:7, "Who serveth as a soldier at any time at his own charges? Who feedeth the flock, and eateth not of the milk of the flock?" Hence it is simoniacal to sell or buy that which is spiritual in such like actions; but to receive or give something for the support of those who minister spiritual things in accordance with the statutes of the Church and approved customs is lawful, yet in such wise that there be no intention of buying or selling, and that no pressure be brought to bear on those who are unwilling to give, by withholding spiritual things that ought to be administered, for then there would be an appearance of simony. But after the spiritual things have been freely bestowed, then the statutory and customary offerings and other dues may be exacted from those who are unwilling but able to pay, if the superior authorize this to be done.
Reply to Objection 1. As Jerome says in his commentary on Micah 3:9, certain gifts were freely offered to the good prophets, for their livelihood, but not as a price for the exercise of their gift of prophecy. Wicked prophets, however, abused this exercise by demanding payment for it.
Reply to Objection 2. Those who give alms to the poor in order to obtain from them the assistance of their prayers do not give with the intent of buying their prayers; but by their gratuitous beneficence inspire the poor with the mind to pray for them freely and out of charity. Temporal things are due to the preacher as means for his support, not as a price of the words he preaches. Hence a gloss on 1 Timothy 5:11, "Let the priests that rule well," says: "Their need allows them to receive the wherewithal to live, charity demands that this should be given to them: yet the Gospel is not for sale, nor is a livelihood the object of preaching: for if they sell it for this purpose, they sell a great thing for a contemptible price." On like manner temporal things are given to those who praise God by celebrating the divine office whether for the living or for the dead, not as a price but as a means of livelihood; and the same purpose is fulfilled when alms are received for making processions in funerals. Yet it is simoniacal to do such things by contract, or with the intention of buying or selling. Hence it would be an unlawful ordinance if it were decreed in any church that no procession would take place at a funeral unless a certain sum of money were paid, because such an ordinance would preclude the free granting of pious offices to any person. The ordinance would be more in keeping with the law, if it were decreed that this honor would be accorded to all who gave a certain alms, because this would not preclude its being granted to others. Moreover, the former ordinance has the appearance of an exaction, whereas the latter bears a likeness to a gratuitous remuneration.
Reply to Objection 3. A person to whom a spiritual power is entrusted is bound by virtue of his office to exercise the power entrusted to him in dispensing spiritual things. Moreover, he receives a statutory payment from the funds of the Church as a means of livelihood. Therefore, if he were to accept anything for the exercise of his spiritual power, this would imply, not a hiring of his labor (which he is bound to give, as a duty arising out of the office he has accepted), but a sale of the very use of a spiritual grace. For this reason it is unlawful for him to receive anything for any dispensing whatever, or for allowing someone else to take his duty, or for correcting his subjects, or for omitting to correct them. On the other hand it is lawful for him to receive "procurations," when he visits his subjects, not as a price for correcting them, but as a means of livelihood. He that is possessed of science, without having taken upon himself the obligation of using it for the benefit of others can lawfully receive a price for his learning or advice, since this is not a sale of truth or science, but a hiring of labor. If, on the other hand, he be so bound by virtue of his office, this would amount to a sale of the truth, and consequently he would sin grievously. For instance, those who in certain churches are appointed to instruct the clerics of that church and other poor persons, and are in receipt of an ecclesiastical benefice for so doing, are not allowed to receive anything in return, either for teaching, or for celebrating or omitting any feasts.
Reply to Objection 4. It is unlawful to exact or receive anything as price for entering a monastery: but, in the case of small monasteries, that are unable to support so many persons, it is lawful, while entrance to the monastery is free, to accept something for the support of those who are about to be received into the monastery, if its revenues are insufficient. On like manner it is lawful to be easier in admitting to a monastery a person who has proved his regard for that monastery by the generosity of his alms: just as, on the other hand, it is lawful to incite a person's regard for a monastery by means of temporal benefits, in order that he may thereby be induced to enter the monastery; although it is unlawful to agree to give or receive something for entrance into a monastery (I, qu. ii, cap. Quam pio).
Source: Summa Theologica (New Advent)