Objection 1. It would seem that simony is not "an express will to buy or sell something spiritual or connected with a spiritual thing." Simony is heresy, since it is written (I, qu. i [Can. Eos qui per pecunias.]): "The impious heresy of Macedonius and of those who with him impugned the Holy Ghost, is more endurable than that of those who are guilty of simony: since the former in their ravings maintained that the Holy Spirit of Father and Son is a creature and the slave of God, whereas the latter make the same Holy Spirit to be their own slave. For every master sells what he has just as he wills, whether it be his slave or any other of his possessions." But unbelief, like faith, is an act not of the will but of the intellect, as shown above (Question 10, Article 2). Therefore simony should not be defined as an act of the will.
Objection 2. Further, to sin intentionally is to sin through malice, and this is to sin against the Holy Ghost. Therefore, if simony is an intentional will to sin, it would seem that it is always a sin against the Holy Ghost.
Objection 3. Further, nothing is more spiritual than the kingdom of heaven. But it is lawful to buy the kingdom of heaven: for Gregory says in a homily (v, in Ev.): "The kingdom of heaven is worth as much as you possess." Therefore simony does not consist in a will to buy something spiritual.
Objection 4. Further, simony takes its name from Simon the magician, of whom we read (Acts 8:18-19) that "he offered the apostles money" that he might buy a spiritual power, in order, to wit, "that on whomsoever he imposed his hand they might receive the Holy Ghost." But we do not read that he wished to sell anything. Therefore simony is not the will to sell a spiritual thing.
Objection 5. Further, there are many other voluntary commutations besides buying and selling, such as exchange and transaction [A kind of legal compromise--Oxford Dictionary]. Therefore it would seem that simony is defined insufficiently.
Objection 6. Further, anything connected with spiritual things is itself spiritual. Therefore it is superfluous to add "or connected with spiritual things."
Objection 7. Further, according to some, the Pope cannot commit simony: yet he can buy or sell something spiritual. Therefore simony is not the will to buy or sell something spiritual or connected with a spiritual thing.
On the contrary, Gregory VII says (Regist. [Caus. I, qu. i, can. Presbyter, qu. iii, can. Altare]): "None of the faithful is ignorant that buying or selling altars, tithes, or the Holy Ghost is the heresy of simony."
I answer that, As stated above (I-II, 18, 2) an act is evil generically when it bears on undue matter. Now a spiritual thing is undue matter for buying and selling for three reasons. First, because a spiritual thing cannot be appraised at any earthly price, even as it is said concerning wisdom (Proverbs 3:15), "she is more precious than all riches, and all things that are desired, are not to be compared with her": and for this reason Peter, in condemning the wickedness of Simon in its very source, said (Acts 8:20): "Keep thy money to thyself to perish with thee, because thou hast thought that the gift of God may be purchased with money."
Secondly, because a thing cannot be due matter for sale if the vendor is not the owner thereof, as appears from the authority quoted (Objection 1). Now ecclesiastical superiors are not owners, but dispensers of spiritual things, according to 1 Corinthians 4:1, "Let a man so account of us as of the ministers of Christ, and the dispensers of the ministers of God."
Thirdly, because sale is opposed to the source of spiritual things, since they flow from the gratuitous will of God. Wherefore Our Lord said (Matthew 10:8): "Freely have you received, freely give."
Therefore by buying or selling a spiritual thing, a man treats God and divine things with irreverence, and consequently commits a sin of irreligion.
Reply to Objection 1. Just as religion consists in a kind of protestation of faith, without, sometimes, faith being in one's heart, so too the vices opposed to religion include a certain protestation of unbelief without, sometimes, unbelief being in the mind. Accordingly simony is said to be a "heresy," as regards the outward protestation, since by selling a gift of the Holy Ghost a man declares, in a way, that he is the owner of a spiritual gift; and this is heretical. It must, however, be observed that Simon Magus, besides wishing the apostles to sell him a grace of the Holy Ghost for money, said that the world was not created by God, but by some heavenly power, as Isidore states (Etym. viii, 5): and so for this reason simoniacs are reckoned with other heretics, as appears from Augustine's book on heretics.
Reply to Objection 2. As stated above (Question 58, Article 4), justice, with all its parts, and consequently all the opposite vices, is in the will as its subject. Hence simony is fittingly defined from its relation to the will. This act is furthermore described as "express," in order to signify that it proceeds from choice, which takes the principal part in virtue and vice. Nor does everyone sin against the Holy Ghost that sins from choice, but only he who chooses sin through contempt of those things whereby man is wont to be withdrawn from sin, as stated above (Question 14, Article 1).
Reply to Objection 3. The kingdom of heaven is said to be bought when a man gives what he has for God's sake. But this is to employ the term "buying" in a wide sense, and as synonymous with merit: nor does it reach to the perfect signification of buying, both because neither "the sufferings of this time," nor any gift or deed of ours, "are worthy to be compared with the glory to come, that shall be revealed in us" (Romans 8:18), and because merit consists chiefly, not in an outward gift, action or passion, but in an inward affection.
Reply to Objection 4. Simon the magician wished to buy a spiritual power in order that afterwards he might sell it. For it is written (I, qu. iii [Can. Salvator]), that "Simon the magician wished to buy the gift of the Holy Ghost, in order that he might make money by selling the signs to be wrought by him." Hence those who sell spiritual things are likened in intention to Simon the magician: while those who wish to buy them are likened to him in act. Those who sell them imitate, in act, Giezi the disciple of Eliseus, of whom we read (2 Kings 5:20-24) that he received money from the leper who was healed: wherefore the sellers of spiritual things may be called not only "simoniacs" but also "giezites."
Reply to Objection 5. The terms "buying" and "selling" cover all kinds of non-gratuitous contracts. Wherefore it is impossible for the exchange or agency of prebends or ecclesiastical benefices to be made by authority of the parties concerned without danger of committing simony, as laid down by law [Cap. Quaesitum, de rerum Permutat.; cap. Super, de Transact.]. Nevertheless the superior, in virtue of his office, can cause these exchanges to be made for useful or necessary reasons.
Reply to Objection 6. Even as the soul lives by itself, while the body lives through being united to the soul; so, too, certain things are spiritual by themselves, such as the sacraments and the like, while others are called spiritual, through adhering to those others. Hence (I, qu. iii, cap. Siquis objecerit) it is stated that "spiritual things do not progress without corporal things, even as the soul has no bodily life without the body."
Reply to Objection 7. The Pope can be guilty of the vice of simony, like any other man, since the higher a man's position the more grievous is his sin. For although the possessions of the Church belong to him as dispenser in chief, they are not his as master and owner. Therefore, were he to accept money from the income of any church in exchange for a spiritual thing, he would not escape being guilty of the vice of simony. In like manner he might commit simony by accepting from a layman moneys not belonging to the goods of the Church.
Source: Summa Theologica (New Advent)