I am currently reading Angela McKay Knobel's Aquinas and the Infused Moral Virtues, on the subject of how the infused virtues relate to the acquired virtues, and it is quite a good discussion of the issues, although I disagree with Knobel's argument in a number of ways, and don't think her account viable. I thought I would say a few things about it.
What we usually call acquired virtues are virtues in the most common sense, namely, they are habits of choice consisting in a mean relative to us as determined by the reason of a prudent person. We get them by repeatedly doing the acts relevant to them; for instance, we become honest by repeatedly doing honest things until it honesty becomes second nature. In the Christian tradition, there is also a general recognition that there are virtues that are similar to these but are not acquired by habituation but by divine grace; the uncontroversial ones are the theological virtues, faith, hope, and love, which as virtues in the proper sense can only be achieved in us by God. Once we have them, we can strengthen them by an analogue of habituation, sometimes called radication -- your faith becomes more ardent or deeply rooted the more you do things so as to grow in it, for instance. Historically, the more controversial question is whether there are infused virtues that are not theological virtues -- the infused moral virtues. Thus, for instance, there is an acquired virtue of honesty; but one can ask if there is a grace-given or infused virtue of honesty (or of some other virtue like it) in addition to this. On some accounts, usually associated with the Scotists, there are no infused moral virtues in the proper sense; what happens, is that we receive the theological virtues and they (especially charity or love) give a new kind of structure to our acquired virtues. The most popular account, though, and to such an extent that I think it's fair to say that it is the generally accepted position even among people who don't adhere to it very consistently, is that there are indeed infused moral virtues, and, indeed, that we have a whole set of infused moral virtues that are like the acquired moral virtues. There is, besides the general human virtue of honesty, for instance, a specifically Christian virtue of honesty, and so forth for every acquired virtue. Besides the ordinary virtues of prudence, justice, fortitude, and temperance, achieved by hard work, there is a specifically Christian version of each, given only by the grace of baptism, lost by sin, restored by repentance (and lost by sin, restored by repentance, lost by sin, restored by repentance...). This is the view of Thomas Aquinas and quite a few others.
This doubling is an interesting feature, and, if discussions of virtue in moral theology are any guide, a very confusing one for most people. In all such matters of confusion, the key question to keep in mind is exactly why one would accept it. After all, things would be simpler if they could be taken out; this is likely one of the motivations for the Scotist position. Thus the most important thing to understand, in order to understand anything about infused moral virtues, is why they are needed in the first place.
Aquinas's argument for them is quite short (ST 2-1.63). He notes that all virtue completes us with respect to some good according to a rule. There are two such rules: human reason and the divine law. Acquired virtues complete the principles of our nature according to the rule of reason. But in giving the theological virtues, God sets us on the path to a higher good than that for which reason alone is appropriate; these theological virtues serve as new principles which then must be completed according to divine law. This is what infused moral virtues are. These infused moral virtues will be analogous to the acquired moral virtues in terms of their structure, but they will in fact not be a mere doubling. There is a generic similarity between, say, acquired temperance and infused temperance, but whereas acquired temperance is concerned with subordinating our eating practices to health and reason, infused temperance is more ascetic because it is concerned with subordinating them to Scripture and God. Further, while acquired virtues tend toward living well in human affairs, the infused virtues tend toward living well in the divine household.
This last point is perhaps more important than it looks, but it ties the whole thing together. Law is always concerned with the common good of some community; natural law is concerned with the common good of the human community, civil law is concerned with the common good of civil society, and divine law is concerned with the common good of the Kingdom of God. Thus our virtues are related to the common goods of the societies of which we are members, an aspect of Aquinas's account that is too often forgotten. We can have human virtues, as we might find among people grown up in contexs without much civilization; these are perfected into civil virtues, with which people participate well in the civilized life of civil society. In addition, different kinds of civil society yield variations in the civil virtues. Aquinas explicitly says in the Disputed Questions on Virtue in General (art. 10) that the relation of the infused moral virtues to the acquired virtues is somewhat like the relation of civil and human virtue; virtue of a human being qua human is directed to civil good by the virtue of a human being qua citizen. Thus the fortitude of the human being qua human is just a general sort of moral toughness according to reason; but if someone develops the fortitude of the human being qua citizen, this is a fortitude for others (namely, one's fellow citizens), and in addition to its own acts, it directs the human virtue of fortitude, that general moral toughness, toward acts concerned with the public good of the city. Likewise, Christians are baptized into the heavenly city, so there are virtues that correspond to being citizens of the heavenly city, which, in addition to their own acts, direct the acts of acquired virtues to the heavenly good. The primary difference between the one side of the analogy and the other is that whereas civil society is a natural perfection of the principles of our nature, heavenly society is not; it is instead a development of grace given to us.
You will notice, first, that 'doubling' is a bit misleading, and something of an understatement: Aquinas thinks that there are lots and lots of virtues, because our virtues have a social aspect and we are not members of only one society; this is the case even if we are only looking at acquired virtues. Every society needs its members to have a virtue of (say) justice; but significantly different societies -- say, tribal societies that are only incipiently civil, Ancient Athens, the modern United States, and the City of God -- will have some variation; in some cases, like the difference between the natural human societies and the divine city, this will be quite considerable. And it is in fact possible for someone to be a member of more than one such society -- as all Christians are with respect to human society and the heavenly society. This is where the apparent doubling happens, not with virtue in general, but specifically with Christians, who are members both of natural societies and of the society of grace. I think that failing to appreciate fully the idea of the social character of virtues that Aquinas inherits from Aristotle is the point at which many people, including Knobel herself, go wrong in considering the relationship of the inherited moral virtues to the acquired moral virtues.
Knobel identifies three general possible ways one might conceive the relation between the two. The first, which she calls the coexistence view, holds that (1) Christians can have both acquired virtues and infused virtues, (2) which each produce acts appropriate to their ends, (3) but in such a way that the acquired virtues receive new direction from the infused virtues to the ends of the infused virtues as well. She argues that these three also require (4) acquired and infused virtues operate in distinct domains of moral life.
The second, which she calls the unification view, holds that in the Christian life, our virtuous actions are more tightly unified than this, either because all virtues in the Christian are infused virtues (which either replace or are themselves just transformed versions of acquired virtues) or because, at least typically, virtuous actions in Christian life are joint actions of acquired virtues and infused virtues.
The third position, her own, attempts to find middle ground between the two. She argues that there remains a distinction between acquired virtues and infused virtues, and that prior habits persist even when one has received infused virtues, but that acquired virtues previously cultivated persist merely as dispositions; acquired virtue in a sense becomes irrelevant, because the whole work of the Christian living a Christian life is to cultivate the infused virtues.
It is, of course, the coexistence view (or at least something very similar) that is actually right. It's the view that most easily fits most of the texts of Aquinas; but more than that (since Aquinas wasn't expositing Aquinas but attempting to characterize a real feature of Christian life) it is really what is required by the Thomistic account of virtue applied to Christian life. Knobel has some arguments that the coexistence view isn't coherent and that it does not fit well with some things Aquinas says about the infused virtues. But the primary question that motivates the arguments that it is not coherent is "Why would it ever be appropriate to merely act in a way proportionate to one's natural fulfillment when one could instead act in a manner proportionate to an adopted child of God and adopted sibling of Christ?" (p. 124). Despite my rejection of Knobel's view, I think this is quite the right question to ask; it's just that I think it's easily answered.
It is, first, somewhat ambiguous. "Merely acting in a way proportionate to one's natural fulfillment" makes it sound like (and Knobel seems to interpret it as) "acting in such a way rather than in a manner proportionate to a child of God". But the coexistence view doesn't commit one to 'rather than'; it commits one to 'as well as'. Knobel comes close to recognizing this in recognizing that for the coexistence view, acquired moral virtues and infused moral virtues have different 'domains', but oddly she almost always interprets the domains as times, that there are times when we act according to acquired virtues and times when we act according infused virtues. This is a tempting but not a great way to interpret it; for one, it's essential to the position that sometimes (perhaps even always) we are, in fact, acting according to both, because we are acting according to acquired virtues given direction by infused virtues. But the distinct domains are not times but societies. And this brings us to the second, and more important point, and the reason why, despite liking much of Knobel's discussion, I absolutely cannot get on board with her proposal. Human beings do not stop being members of human society by becoming members of the divine society; they do not stop being members of their civil societies by becoming members of the Kingdom of God. I did not cease to be an American citizen when I was baptized. My participation in American society still continues. To be sure, it is and should be subordinate to my participation in the heavenly society; my American citizenship became part of my pilgrimage in this world, but I am still a member of that society, and of the general human community, and need to work on the virtues appropriate to those. It's not that there are times to be an American and times to be a Christian; I'm actually supposed to be both all the time. Some things may be more Americanish things, and other things may be more Christianish things, but everything I do needs to be Christian, including the American things, and I need actually to be American in doing American things. The view we find in the New Testament is not one of absolute severance from human society; we do not become angels, we continue to be men and need to respect the king and magistrates operating in their legitimate spheres, and, like Paul, who remained both Jew and Roman, none of us renounce our earthly citizenships and allegiances even in gaining a superior heavenly one. A view in which Christians only cultivate infused virtues is a view in which Christians stop being human; it would perhaps be good for a Gnostic movement, but it does not fit well with the actual faith.
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Angela McKay Knobel, Aquinas and the Infused Moral Virtues, University of Notre Dame Press (Notre Dame, IN: 2021).
