[Sloth] rob[s] us of our appetite for God, our zest for God, our interest and enjoyment in God. Sloth stops is from seeking God, and that means we do not find him. . . . It may seem strange to define a mortal sin as a kind of sorrow, for sorrow is in itself only a feeling, and feelings are neither good nor evil. But sloth is our applying this feeling to something, to our highest end. Feelings come to me, and I am not directly responsible for their coming; but I am responsible for their going, for what I do with them, inwardly as well as outwardly. When I am sorrowful about my divine good, when my soul says No to God’s offer of supreme joy, when I return his invitation ticket to his banquet, I am spiritually dead. Sloth is a cold sin, not a hot one; but that makes it even deadlier. Rebellion against God is closer to him than indifference is, just as hatred is closer to love than indifference is. God can more easily cool our wrath than fire our frozenness, though he can do both. Sloth is a sin of omission, not commission.
—Peter Kreeft, Back to Virtue, pp. 153-154