The tribulations, to which the people of God are subject, are internal, as well as external; sorrows of the mind as well as sufferings of the body. Sometimes they are very great. There are some occasions, on which all those subordinate consolations, of which God generally permits his people in a greater or less degree to partake, are taken away. There is left to them neither the vivacity of health nor the consolation of friends; no pleasures of social intercourse; no prosperity in worldly business; no rest from outward persecutions; no cessations from the bitter temptations of the adversary. This, it will be said, is an extreme case; but it is only extreme cases, of which, in the present chapter, we propose to speak. There is reason to suppose, that many souls, whom God designs to bring to the highest degree of purity in this life, especially if they are disposed to resist and do not render themselves up easily to his great purpose, will be called upon to pass through some heavy and perhaps extreme trials. Such trials seem oftentimes to be rendered necessary. Necessary not in the nature of things, but on account of the corruption of the natural heart. The possession of internal purity implies the entire crucifixion of self; and this is an operation which the natural heart finds it hard to submit to. Hence it is, that earthly joys are temporarily dried up; that human consolations are taken away; and "the axe is laid at the root" of all the sources of self-seeking and self-enjoyment; in order that the soul may experience the truth and the severity of inward crucifixion.
It is at such a time, and amid these various and unmitigated trials, that the soul sustains itself by FAITH; by what is variously called in different writers, but generally as I suppose with the same meaning, "simple faith", "pure faith", or "naked faith." And there seems to be a marked propriety in these forms of expression; because faith, as the sustaining principle, stands at such times alone. All human supports are removed. On every side there appears discouragement and darkness; and it is by faith and faith only, that the soul is enabled to retain its religious integrity. It is under such circumstances, that faith becomes, as it were, a superior and guiding faculty of the soul; upon which the others, especially the various inferior principles, seem to rest. While the subordinate principles of our nature, the natural desires, and the various forms of natural affection, are assailed by their appropriate temptations, and sometimes in a very severe and terrible manner, they derive from the sublime principle of faith, which stands in its central position of strength and grandeur, a defensive and repulsive power, which makes them more than conquerors.
— edited from The Interior or Hidden Life (1844) Part 1, Chapter 18.