The following is an excerpt of a discussion by John of the Cross (1542-1591) in his book Dark Night of the Soul. He is discussing "the seven capital sins" of the spiritual life. He has already discussed spiritual pride, spiritual greed, spiritual luxury, spiritual wrath, and spiritual gluttony. Now he will discuss the final two: spiritual envy and spiritual sloth.
The last two sins are the vices of spiritual envy and spiritual sloth. People who fancy themselves as spiritual are quite often not pleased to hear about the spiritual growth of others. Their chief concern is to be praised themselves. They are not pleased that such attention is being given to someone else and would prefer to be thought of as the most spiritual of all. This is contrary to love, which as Paul says rejoices in goodness.
Spiritual sloth happens when the pleasure is removed from the spiritual life. Such souls become weary with spiritual exercises because they do not yield any consolation, and thus, they abandon them. They become angry because they are called to do that which does not fit their needs. They begin to lose interest in God for they measure God by themselves and not themselves by God. Such souls are too weak to bear the crosses that we are given to us to help us grow, crosses we face in the dark night of the soul.
(Cited in Richard Foster’s Devotional Classics, p. 36)