Darkness Within Faith, Blessed Teresas of Calcutta
Today was the 10th anniversary of the death of Blessed Teresa of Calcutta. Though we have known of her darkness for some time now, a new book that has come out, Come Be My Light, detailing her spiritual agony through the many letters she wrote to spiritual directors through the decades. Many report it as some sort of double life she was leading. They cannot fathom the idea of this kind of spiritual struggle from one who acted as if always in union with the Divine. But darkness and faith very often go hand in hand.
Probably the most famous author on spiritual darkness is the 16th century Spanish mystic, St. John of the Cross, with his Dark Night of the Soul. 
But modern saints have written of their experience as well. My favorite, St. Terese of Lisieux (1873-1897), a doctor of the Church for her Little Way of spiritual childhood, suffered what she called “worst temptations of atheism” for the last 18 months of her life. During this time she says God
“permitted my soul to be invaded by the thickest darkness, and that the thought of heaven, up until then so sweet to me, be no longer anything but the cause of struggle and torment.”
For her the veil of faith turned into a wall reaching “right up to the heavens” covering the starry firmament.
When I sing of the happiness of heaven and of the eternal possession of God, I feel no joy in this, for I sing simply what I WANT TO BELIEVE.
I have not yet read the book on Mother Teresa, only the Time magazine article, but from what I have read, the words she speaks in her darkness are very similar, though hers seems considerably more painful:
When I try to raise my thoughts to Heaven – there is such convicting emptiness that those very thoughts return like sharp knives & hurt my very soul. – I am told God loves me – and yet the reality of the darnkness and coldness and emptiness is so great that nothing touches my soul.
The saints recognize this trial as being permitted by God and taken away only at an “hour set by God Himself” (St. Therese). 
St. Maria Faustina (1905-1938), who I have just begun to read, offers further insight into these trials and God’s relationship with the chosen souls. Though the soul feels abandoned by God, He is still there, perhaps closer than ever, especially in time of temptation:
“God supports the soul in secret, so to speak. The soul is not aware of this, but otherwise it would be impossible to stand firm; and God knows very well how much he can allow to befall the soul.”
It is quite difficult to put such spiritual suffering into words, and one must “travel through this dark tunnel to understand its darkness” (St. Therese). What we can tell, however, from the example of the saints is that God brings such darkness to a soul because it has been “admitted to a deeper intimacy with Him” and “He wants to try it as pure gold is tried” (St. Faustina) We may not all be called to experience the Trial of Trials, that of complete abandonment and despair, but we will inevitably face some challenges to our faith. For those challenges we can now look to another great example of love and perseverance. Though we don’t always have the joy of faith, we should “carry out its works at least” (St. Therese).
“In this you rejoice, though now for a little while you may have to suffer various trials, so that the genuineness of your faith, more precious than gold which though perishable is tested by fire, may redound to praise and glory and honor at the revelation of Jesus Christ.” (1 Pet 1:6-7)
Suggested reading:
Come Be My Light
Story of a Soul: The Autobiography of St. Therese of Lisieux
St. Therese of Lisieux: Her Last Conversations
Diary of Saint Maria Faustina Kowalska