Before going on I want to say that you should consider what it would mean to this so brilliantly shining and beautiful castle, this pearl from the Orient, this tree of life planted in the very living waters of life [Cf. Ps 1:3]—that is, in God—to fall into mortal sin; there’s no darker darkness nor anything more obscure and black.
You shouldn’t want to know anything else than the fact that, although the very sun that gave the soul so much brilliance and beauty is still in the center, the soul is as though it were not there to share in these things. Yet, it is as capable of enjoying His Majesty as is crystal capable of reflecting the sun’s brilliance.
Nothing helps such a soul, and as a result all the good works it might do while in mortal sin are fruitless for the attainment of glory.
Since these works do not proceed from that principle, which is God, who is the cause of our virtue being really virtue, and are separated from Him, they cannot be pleasing in His sight. Since, after all, the intention of anyone who commits a mortal sin is to please the devil, who is darkness itself, not God, the poor soul becomes darkness itself.
Saint Teresa of Avila
The Interior Castle, I, ch. 2, no. 1
Teresa of Avila, St. 1985, The Collected Works of St. Teresa of Avila, translated from the Spanish by Kavanaugh, K; Rodriguez, O, ICS Publications, Washington DC.