In this first stanzas the soul sings of the happy fortune and chance which it experienced in going forth from all things that are without, and from the desires and imperfections that are in the sensual part of man because of the disordered state of his reason.
For the understanding of this it must be known that, for a soul to attain to the state of perfection, it has ordinarily first to pass through two principal kinds of night, which spiritual persons call purgations or purifications of the soul.
And here we call them nights, for in both of them the soul journeys, as it were, by night, in darkness. The first night or purgation is of the sensual part of the soul…and the second is of the spiritual part.
And this first night pertains to beginners, occurring at the time when God begins to bring them into the state of contemplation….And the second night, or purification, pertains to those who are already proficient, occurring at the time when God desires to bring them to the state of union with God.
[…] Briefly, then, the soul means by this stanza that it went forth (being led by God) for love of Him alone, enkindled in love of Him, upon a dark night, which is the privation and purgation of all its sensual desires, with respect to all outward things of the world and to those which were delectable to its flesh, and likewise with respect to the desires of its will.
This all comes to pass in this purgation of sense; for which cause the soul says that it went forth while its house was still at rest; which house is its sensual part, the desires being at rest and asleep in it, as it is to them.
For there is no going forth from the pains and afflictions of the secret places of the desires until these be mortified and put to sleep.
And this, the soul says, was a happy chance for it — namely, its going forth without being observed: that is, without any desire of its flesh or any other thing being able to hinder it.
And likewise, because it went out by night — which signifies the privation of all these things wrought in it by God, which privation was night for it.
And it was a happy chance that God should lead it into this night, from which there came to it so much good; for of itself the soul would not have succeeded in entering therein, because no man of himself can succeed in voiding himself of all his desires in order to come to God.
John of the Cross (1542-1591): Ascent of Mount Carmel, 1,1.