What is the Dark Night of the Soul?

"The soul, however, cannot be perfectly purified from these imperfections, any more than from the others, until God shall have led it into the passive purgation of the dark night, of which I shall speak immediately. But it is expedient that the soul, so far as it can, should labor, on its own part, to purify and perfect itself, that it may merit from God to be taken under His divine care, and be healed from those imperfections which of itself it cannot remedy. For, after all the efforts of the soul, it cannot by any exertions of its own actively purify itself so as to be in the slightest degree fit for the divine union of perfection in the love of God, if God Himself does not take it into His own hands and purify it in the fire, dark to the soul."

— St. John of the Cross, Dark Night of the Soul, p.14