Love requires a decree of the will; but when the will has decreed, love does not reach perfection all at once. It requires time. It kindles little by little, until, by continued fanning, it bursts at last into a flame. So with the love of virtue and happiness wholly divested of their accidental surroundings. First, a thoroughly purified knowledge of their nature must be fixed in the mind, and this takes a very long time. Then there must come the volition, strong and determined, and this also is a thing that cannot be withdrawn from the laws of time. Only after this, that is to say, after very protracted and oft-repeated acts and efforts, can the love of pure virtue and happiness rise to that height of fortitude which gives it strength to overcome all the allurements of sensible things. Such at least is the ordinary course which love pursues, if not in each individual, certainly in humanity at large. A long time, therefore, must have elapsed before it could run through all this course, and so reach perfection in the end.
Rosmini, Theodicy, Volume 1, pp. 327-328.
This is part of Rosmini's account of moral progress. Any human being in any state of human civilization can devote themselves to virtue, but love presupposes knowledge, and so the ability to love virtue as such, rather than in this or that particular form, requires that virtue's essential features have already been distinguished out and conceptualized. Thus people in crude societies can be virtuous and pursue virtue, but they have difficulty distinguishing between virtue itself and this or that particular form in which the virtue happens to express itself in their society. Thus, for instance, moral happiness tends to be thought of in terms of material happiness -- but also vice versa -- and virtue tends to be thought of as requiring a particular customary shape.
A good context for thinking about this, to use a different kind of example from those which Rosmini uses, might be Beowulf. In Beowulf, justice is depicted entirely in material terms: the just king is one who bravely and generously gains wealth for his people. It is not a materialistic conception, because it is genuinely about justice rather than the actual wealth, but the just acts are understood only in this particular form of gathering wealth and distributing it for the good of everyone. The audience for Beowulf would have had difficulty grasping the notion of a king who did not do this in particular and yet was somehow still just, not because they did not know what justice was (they could recognize it in this case), but because they could not fully abstract justice as such from this particular just thing to which they were accustomed, and so tended to treat the general and particular as equivalent.
Once society's development has reached a point that people can recognize the essential features of virtue and distinguish them from the accidental features it has in concrete particular expressions, they can pursue virtue itself and purely for its own sake. (But they also run the opposite danger of overspiritualizing this pursuit and treating it as if virtue could actually be pursued without regard for the features it has in concrete particular expressions. Thus Rosmini says that primitive societies tend to err in moral matters by not making enough distinctions, while modern societies tend to err in moral matters by making too many.) Thus Rosmini insists that human ethics by its nature requires refinement, and this refinement can be quite difficult and take a very long time. A jillion years ago in primitive circumstances, human beings could be just and act justly; but only much experience in these just acts (and, as it happens, much experience of people acting unjustly) could help them be clear about what actually is necessary for just acts in general to be just.
(All of this is complicated a bit by the fact that moral progress is not inevitable, at least not by human means alone; we can degenerate as well as develop, and particular societies themselves can go through cycles of progress and decline. Relatedly, it is complicated by the fact that each generation needs actively build on the discoveries of their predecessors in order to progress at all, so each generation must usually, although with previously discovered shortcuts and the help of more experienced prior generations, learn it all over again. We can all recognize justice and injustice to some extent, but our reasoning depends in great measure on our vocabulary for talking about them, which means that we must be taught the language, or else discover it the long and hard way ourselves.)
Rosmini has an interesting follow-up argument to this. How, if they are not fully conceptualizing justice in its proper form, do people actually practice justice? Rosmini's answer is that this happens through sacrificial systems, in which symbolic exchanges yield representations that can be used in moral reasoning, and through the family, which serves as the primitive community in which we develop and which can then serve as a basis for analogy to broader concerns. Thus our various basic pieties, whether religious or filial, serve as initial templates for virtue generally, something that can start us off and be refined, and he takes it to be the case that this is in fact what providential history shows.
