Faith and Acquired vs. Infused Habits in Aquinas

Faith introduces into our lives an element inaccessible to reason alone, namely, the pure truth of God as He truly is, God as a Trinity of love. God speaks His Word to us (John 1), and this Word is truth (John 14:6) that has come to set us free (John 8:32). We hear the Word (Romans 10:17) and either accept Him or reject Him. We cannot accept only part of Christ, part of the Word. We must either accept all of Him or reject all of Him, just as the eye cannot say to the hand, "I do not need you" (1 Corinthians 12:21). The entire body works as a coherent whole, and every part is integral.

By faith alone do we enter into this integrated whole of revelation. This is the fundamental reason why one cannot be a "Cafeteria Christian"; the very notion is self-contradictory. A Christian is a follower of Christ, one who accepts the Word of God or at least strives to. We cannot reason our way to this acceptance; we cannot understand it like we can understand the biology or chemistry of a physical object. It must be revealed to us as it is, and while we can reflect on it and draw out its implications, necessary or possible, we hold onto these truths in a certain intellectual darkness or obscurity.

Faith is an infused virtue, a power of the soul that God gives us. Two people can witness the same miracle, and only one might convert. Why? Because God gives the grace of faith to one and not the other, for only by grace can one make an act of belief. St. Paul said so explicitly: "For by grace you have been saved through faith, and this is not from you; it is the gift of God; it is not from works, so no one may boast" (Eph. 2:8-9).

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Fr. Jordan Aumann, O.P., Spiritual Theology, chapter 4: "The Supernatural Organism"; Nature of the Infused Virtues, 81-83:

The infused virtues may be defined as operative habits infused by God into the faculties of the soul to dispose them to function according to the dictates of reason enlightened by faith.

"Operative habits" is the generic element of the definition, common to all natural and supernatural virtues. On the purely natural level an operative. habit is a quality, difficult to remove, that disposes the subject to function with facility, promptness, and delight. It gives the subject facility for operation because every habit is an increase of energy in relation to its corresponding action; it gives promptness because it constitutes, so to speak, a second nature in virtue of which the subjects quickly give themselves to action; and it causes delight in the operation because it produces an act that is prompt, facile, and connatural.

"Infused by God" is a radical difference between the infused and acquired virtues. The natural or acquired virtues are engendered in us by means of repeated acts; the only cause of the supernatural or infused virtues is the divine infusion. Their purpose is to supernaturalize the faculties by elevating them to the order of grace and making them capable of performing supernatural acts. Without them, or without the actual grace that substitutes for them (as in the case of the sinner before justification), it would be impossible for us to perform a supernatural act. St. Thomas says: "As from the essence of the soul flows its powers, which are the principles of deeds, so likewise the virtues, whereby the powers are moved to act, flow into the powers of the soul from grace."

The principal difference between the acquired and infused virtues is by reason of the formal object. The infused virtues dispose the faculties to follow the dictate or command, not of reason alone, as do the acquired virtues, but of reason illumined by faith. The acquired moral virtues, however heroic and perfect, could never attain the formal object of the infused virtues. With good reason does St. Thomas say that the principal difference between the acquired and infused virtues is by reason of their formal objects:

The object of every virtue is a good considered as in that virtue's proper matter; thus the object of temperance is a good with respect to the pleasures connected with the concupiscence of touch. The formal aspect of this object is from reason, which fixes the mean in these concupiscences. Now it is evident that the mean that is appointed in such concupiscence according to the rule of human reason is seen under a different aspect from the mean that is fixed according to the divine rule. For instance, in the consumption of food, the mean fixed by human reason is that food should not harm the health of the body nor hinder the use of reason; whereas according to the divine rule it behooves man to chastise his body and bring it under subjection (1 Cor. 9:27) by abstinence in food, drink, and the like. It is therefore evident that infused and acquired temperance differ in species; and the same applies to the other virtues.

Nor does it change matters to object that the act of infused temperance is identical with that of acquired temperance (namely, the moderation or control of the pleasures of touch) and that therefore there is no specific difference between them. St. Thomas admits the identity of the material object but insists on the specific and radical difference by reason of the formal object: "Both acquired and infused temperance moderate desires for pleasures of touch, but for different reasons as stated: wherefore their respective acts are not identical." [...]

The principal differences between the acquired and infused virtues can be summarized as follows:

By reason of their essence. The natural or acquired virtues are habits in the strict sense of the word. They do not give the power to act (for the faculty has that already), but they give facility in operation. The supernatural or infused virtues give the power to act supernaturally (without them it would be impossible, apart from an actual grace), but they do not give facility in operation.
By reason of the efficient cause. The natural virtues are acquired by our own proper acts; the supernatural virtues are infused by God together with sanctifying grace.

By reason of the efficient cause. The natural virtues are acquired by our own proper acts; the supernatural virtues are infused by God together with sanctifying grace.

By reason of the final cause. The acquired, natural virtues enable us to conduct ourselves rightly in regard to human acts in accordance with our rational nature. The supernatural virtues, on the other hand, give us the ability to conduct ourselves rightly in regard to our condition as adopted children of God, destined for eternal life, and to exercise the supernatural acts. proper to the life of grace:

By reason of the formal object. The natural virtues work for the good according to the dictate and light of natural reason; the supernatural virtues work for the good according to the dictate and supernatural light of faith.