as explicated by St. John of the Cross:
from Dark Night of the Soul I:11:11-12
St. John of the Cross
11. Finally, insofar as these person are purged of their sensory affections and appetites, they obtain freedom of spirit in which they acquire the twelve fruits of the Holy Spirit.
They are also wondrously liberated from the hand of their enemies, the devil, the world, and the flesh. For when the sensory delight delight and gratification regarding things is quenched, neither the devil, nor the world, nor sensuality has arms or power against the spirit.
12. These aridities, then, make people walk with purity in the love of God. No longer are they moved to act by the delight and satisfaction they find in a work, as perhaps they were when the derived this from their deeds, but by the desire of pleasing God. They are neither presumptuous nor self-satisfied, as was their custom int he time of their prosperity, but fearful and disquieted about themselves and lacking in any self-satisfaction. This is the holy fear that preserves and gives increase to the virtues.
I am not original in claiming that the dark night had for Blessed Mother Teresa a protective effect, an effect all the more necessary in a world where the entire world is at your doorstep and scrutinizing every action.
This deep and unsatisfied longing for God's presence has the unique attribute of taking away from her the many temptations that come as a result of success in the world. Satan's most effective ploy in dealing with someone like Mother Teresa would be to have them change their focus from serving and saving souls to better the lives of people. These two sound like hand in glove; however, they are as different in focus as a microscope and a telescope.
What if Mother Teresa, not wandering in a dark night of spirit had started to pay more attention to things that mattered, but were no the One Thing. What if she suddenly started to say to herself, "With a few dollars more, I could build a house for twenty more people." What is the focus of her effort became the betterment of lives through better buildings, more technology, what have you, rather than helping people to get what they needed to live a life and leave a life with dignity. No matter how holy the motive, when the focus slips from, "For God and God alone, a gift of His people," to "Look what we can do if we only try," Satan has won.
But the dark night has a paradoxical effect. The longing for and the apparent absence of God in a life, increases the focus on serving Him. It cocoons the person away from some of the yammerings of the world and helps them to see life as it should be seen.