14 But why have I, as if forgetting to whom I address myself, assumed the tone of a teacher in stating the question regarding which I wish to be instructed by you? Nevertheless, as I had resolved to submit to your examination my opinion regarding the equality of sins (a subject involving a question closely bearing on the matter on which I was writing), let me now at last bring my statement to a conclusion. Even though it were true that he who has one virtue has all virtues, and that he who lacks one virtue has none, this would not involve the consequence that all sins are equal; for although it is true that where there is no virtue there is nothing right, it by no means follows that among bad actions one cannot be worse than another, or that divergence from that which is right does not admit of degrees. I think, however, that it is more agreeable to truth and consistent with the Holy Scriptures to say, that what is true of the members of the body is true of the different dispositions of the soul (which, though not seen occupying different places, are by their distinctive workings perceived as plainly as the members of the body), namely, that as in the same body one member is more fully shone upon by the light, another is less shone upon, and a third is altogether without light, and remains in the dark under some impervious covering, something similar takes place in regard to the various dispositions of the soul. If this be so, then according to the manner in which every man is shone upon by the light of holy love, he may be said to have one virtue and to lack another virtue, or to have one virtue in larger and another in smaller measure. For in reference to that love which is the fear of God, we may correctly say both that it is greater in one man than in another, and that there is some of it in one man, and none of it in another; we may also correctly say as to an individual that he has greater chastity than patience, and that he has either virtue in a higher degree than he had yesterday, if he is making progress, or that he still lacks self-control, but possesses, at the same time, a large measure of compassion.
15. To sum up generally and briefly the view which, so far as relates to holy living, I entertain concerning virtue—virtue is the love with which that which ought to be loved is loved. This is in some greater, in others less, and there are men in whom it does not exist at all; but in the absolute fullness which admits of no increase, it exists in no man while living on this earth; so long, however, as it admits of being increased there can be no doubt that, in so far as it is less than it ought to be, the shortcoming proceeds from vice. Because of this vice there is “not a just man upon earth that does good and sins not;” because of this vice, “in God's sight shall no man living be justified.” On account of this vice, “if we say that we have no sin, we deceive ourselves, and the truth is not in us.” On account of this also, whatever progress we may have made, we must say, “Forgive us our debts,” although all debts in word, deed, and thought were washed away in baptism. He, then, who sees aright, sees whence, and when, and where he must hope for that perfection to which nothing can be added. Moreover, if there had been no commandments, there would have been no means whereby a man might certainly examine himself and see from what things he ought to turn aside, whither he should aspire, and in what things he should find occasion for thanksgiving or for prayer. Great, therefore, is the benefit of commandments, if to free will so much liberty be granted that the grace of God may be more abundantly honoured.
Source: Letters (New Advent)