Commentary Purg XVII 127-132

Virgil now defines Sloth, the first of these two better kinds of sin, as 'laggard love.'  Thus, by failing to respond to God's offered love more energetically, the slothful are more rebellious to Him than are the avaricious, gluttonous, and lustful, who are pursuing a secondary good rather than avoiding the primary one.  In Dante's day, Sloth was a sin particularly identified with the clergy, with reference not so much to their physical laziness as to their spiritually laggard lives.  See Kuhn (Kuhn.1976.1), pp. 39-64.