Explains the third  line of the second stanza.
IT is very clear that it was a happy chance for this soul to go forth with such an enterprise as this, for it was its going forth that delivered it from the devil and from the world and from its own sensuality, as we have said. Having attained liberty of spirit, so precious and so greatly desired by all, it went forth from low things to high; from terrestrial, it became celestial; from human, Divine. Thus it came to have its conversation in the heavens, as has the soul in this state of perfection, even as we shall go on to say in what follows, although with rather more brevity.
2. For the most important part of my task, and the part which chiefly led me to undertake it, was the explanation of this night to many souls who pass through it and yet know nothing about it, as was said in the prologue. Now this explanation and exposition has already been half completed. Although much less has been said of it than might be said, we have shown how many are the blessings which the soul bears with it through the night and how happy is the chance whereby it passes through it, so that, when a soul is terrified by the horror of so many trials, it is also encouraged by the certain hope of so many and such precious blessings of God as it gains therein. And furthermore, for yet another reason, this was a happy chance for the soul; and this reason is given in the following line:
In darkness and in concealment.
 i.e., in the original Spanish and in our verse rendering of the poem in The Complete Works of St. John of the Cross, Ed. by E. Allison Peers, Vol. II (The Newman Press, Westminster, Md.).