In discussing what grace is, Aquinas raises an objection that it cannot be a habit, because it is easily lost: "if grace is something in the soul, it seems that it must be a habit… but it is not a habit, for a habit is a quality that is not easily altered, as the Philosopher says in the Categories, while grace is removed with the greatest ease, since it is lost by one act of mortal sin. Therefore grace is not something in the soul" (De Veritate, q. 27, a. 1, obj 9).
In response, Aquinas denies the premise of the objection, that grace is easily lost. "Although grace is lost through a single act of mortal sin, grace is not easily lost; for it is not easy for the person who has grace to do such an act, on account of his contrary inclination, as Aristotle says in Ethics V, that it is difficult for the just man to do unjust deeds."