What underlies both reactions is a failure to understand an experience as old as faith itself, that of being inside a dark night of the soul. Looking at Mother Theresa through the eyes of Christian mysticism the better question might be: How could she not experience what she experienced? She was an extraordinary woman, a spiritual athlete, someone who had given her entire freedom over to God; might we not expect this to happen to her? Wouldn't you expect her to experience a dark night of the soul?
What is a dark night of the soul? A dark night of the soul is an experience where our felt-sense of God dries up and disappears. At the level of feeling, thought, and imagination, we are unable to conjure up any sense of security or warm feelings about the presence of God in our lives. We feel agnostic, even atheistic, because we can no longer imagine the existence of God. God seems non-existence, absent, dead, a fantasy of wishful thinking.
But notice that this takes place at the level of the imagination and feelings. God doesn't disappear or cease to exist. What disappears are our former feelings about God and our capacity to imagine God's existence.
God exists, independent of our feelings. Sometimes our heads and hearts are in tune with that and we feel its reality with fervor. Other times our heads and hearts cannot attune themselves to the think, imagine, and feel the existence of a God who ineffable, unimaginable, and Other (by definition) and we experience precisely a certain absence, depression, or void when we try to imagine God's existence and love.
We should expect this in our lives; Jesus experienced dark nights of the soul. Just before he died on the cross, he cried out in anguish, expressing feelings of being abandoned by God. But inside this seeming agnosticism something beyond his feelings and imagination held him steady and enabled him to give himself over in trust to Someone whom he could no longer imagine as existing. This wasn't doubt, it was real faith. Faith begins exactly where atheism assumes it ends.
If this happened to Jesus, should we be surprised that it happened to Mother Theresa. Henri Nouwen tells how shocked and surprised he was at the deathbed of his mother, a woman of extraordinary, when she began to express anguish and feelings of abandonment by God: "How can this be happening to my mother?" Later, upon reflection, it made sense. His mother had prayed every day of her adult life to die like Jesus. God simply took her prayer and her offer seriously.
Understood correctly a dark night is not a failure in faith but a failure in our imagination: Imagine sitting down to pray one day and having the sure sense that God is real, more real in fact than anything else. At that moment, your faith feels secure both in your head and in your heart. Then imagine a different scene: You are lying in bed, in the dark, one night and, with every ounce of sincerity, intelligence, and will-power, you try to imagine and feel God's existence and come up empty and dry. You are haunted by the fear: "I don't believe! Deep down I'm an atheist!" Does this mean that in the one instance you had strong faith and in the next you had weak faith?
Not necessarily. In the first instance you had a strong imagination and in the second you had a weak one. In one instance, you were able to imagine the existence of God and the other you weren't. Neither determines whether God exists or not. Dark nights of faith have to do with feelings and the imagination and not with God's reality or presence to us.
Why are dark nights of faith given to us? Why does God seemingly sometimes withdraw his presence? Always to make us let go of something that, while it may have been good for awhile, an icon, is now causing some kind of idolatry in our lives.
Whenever we cry out to in faith and ask God why he isn't more deeply present to our sincerity, God's answer is always the same one he gives in Scripture, time and time again: You will find me again when you search for me with your whole heart, your whole mind, and your whole soul, that is, when you let go of all the things that, right now, in your mind and heart you have mistaken for God!