Acquired virtue does not make us avoid sin always, but only for the most part; for it is also true of natural occurrences that they happen for the most part. It does not follow from this that someone is both virtuous and vicious, because a single action is not enough in a capacity to remove the disposition of a vice or of an acquired virtue. Also, one cannot avoid all sin through an acquired virtue; for acquired virtues do not save us from the sin of lack of faith, or from the other sins that are opposed to the infused virtues.
Thomas Aquinas, Disputed Questions on the Virtues (q. 1, a.9, ad 5), Atkins and Williams, ed. CUP (New York: 2005) p. 57.
