Mysticism and Christianity
It is characteristic of mysticism that it makes its appeal to the feelings as the sole, or at least as the normative, source of knowledge of divine things. That is to say, it is the religious sentiment which constitutes for it the source of religious knowledge. Of course mystics differ with one another in the consistency with which they apply their principle. And of course they differ with one another in the account they give of this religious sentiment to which they make their appeal. There are, therefore, many varieties of mystics, pure and impure, consistent and inconsistent, naturalistic and supernaturalistic, pantheistic and theistic—even Christian. What is common to them all, and what makes them all mystics, is that they all rest on the religious sentiment as the source of knowledge of divine things.
The great variety of the accounts which mystics give of the feeling to which they make their appeal arises from the very nature of the case. There is a deeper reason for a mystic being "mute" — that is what the name imports—than that he wishes to make a mystery of his discoveries. He is "mute" because, as a mystic, he has nothing to say. When he sinks within himself he finds feelings, not conceptions; his is an emotional, not a conceptional, religion; and feelings, emotions, though not inaudible, are not articulate. As a mystic, he has no conceptional language in which to express what he feels. If he attempts to describe it he must make use of terms derived from the religious or philosophical thought in vogue about him, that is to say, of non-mystical language. His hands may be the hands of Esau, but his voice is the voice of Jacob. The language in which he describes the reality which he finds within him does not in the least indicate, then, what it is; it is merely a concession to the necessity of communicating with the external world or with his own more external self. What he finds within him is just to his apprehension an "unutterable abyss." And Synesius does himself and his fellow mystics no injustice when he declares that "the mystic mind says this and that, gyrating around the unutterable abyss."
(The Works of Benjamin B. Warfield, Volume IX, p. 651-652).