I know of a person30 to whom Our Lord wished to show what a soul was like when it committed mortal sin. That person says that, if people could understand this, she thinks they would find it impossible to sin at all, and, rather than meet occasions of sin, would put themselves to the greatest trouble imaginable. So she was very anxious that everyone should realize this. May you be no less anxious, daughters, to pray earnestly to God for those who are in this state and who, with all their works, have become sheer darkness. For, just as all the streamlets that flow from a clear spring are as clear as the spring itself, so the works of a soul in grace are pleasing in the eyes both of God and of men, since they proceed from this spring of life, in which the soul is as a tree planted. It would give no shade and yield no fruit if it proceeded not thence, for the spring sustains it and prevents it from drying up and causes it to produce good fruit. When the soul, on the other hand, through its own fault, leaves this spring and becomes rooted in a pool of pitch-black, evil-smelling water, it produces nothing but misery and filth.
30 St. Teresa herself. See Relation XXIV (Vol. I, The Complete Works of St. Teresa, translated and edited by E. Allison Peers, p. 345).