Section — Slater, Manual of Moral Theology
6. Sloth
1.
Spiritual sloth is a sluggishness of the soul in the exercise of virtue. If the reason for the sluggishness is the labour and difficulty which accompany the practice of virtue, sloth will be a mortal sin whenever on account of it a grave precept is violated; otherwise it will be a venial sin. If sloth makes the friendship of God tedious and irksome because of the trouble it takes to preserve it, it is a mortal sin, inasmuch as it is directly against our obligation of loving God with our whole heart.
p. i, 102