Mother Theresa of Calcutta also said something like this. Her words: "Once you've got God within you, that's for life. There is no doubt. You can have other doubts, eh? But that particular one will never come again."
According to one book about Mother Teresa, she had many doubts, but not an ultimate doubt. As I've read about this aspect of her life, I feel that the authors may do her and Truth a little disservice. She didn't seem to emphasize this doubt, but lived in faith of that which she knew to be true.
And I'd like to go a step further with this. Once we realize what is ultimately true...(we could use the word God here, as Mother Teresa would, but I prefer some less abused, more open-ended words like Buddhist translations of Nibbana. Naming only a few: Ultimate Reality, the Deathless, Truth, emptiness, suchness...)
Once we realize what is ultimately true, darkness is no longer a bad word, not a parallel of wrong, bad, or evil. It is open space. We may stumble upon and embrace a darkness that is pure faith and includes, even requires, release of reliance on self, or who or what we identify with as self.
If we are lucky this stumbling upon may be more like being grasped by deep hunger or even as a sense of being deeply loved. We may cling to the love as if it were a solid thing, ours to own, rather than opening to the great unknown, realizing love is a living flowing tide that only lives and grows as it flows.
Is it Not Time
Is it not time
to free ourselves from the beloved
even as we trembling, endure the loving?
As the arrow endures the bowstring's tension
so that, released, it travels farther.
For there is nowhere to remain.
References to Mother Teresa from Mother Teresa: Come Be My Light - The Private Writings of the Saint of Calcutta