True quietness of soul involves a cessation from unnecessary wandering and discursive thoughts and imaginations.
If we indulge an unnatural and inordinate curiosity; if we crowd the intellect not only with useful knowledge, but with all the vague and unprofitable rumors and news of the day, it is hardly possible, on the principles of mental philosophy, that the mind should be at rest. The doctrine of religious quietude conveys the notion of a state of intellect so free from all unnecessary worldly intruders, that God can take up his abode there as the one great idea, which shall either exclusively occupy the mind, or shall so far occupy it as to bring all other thoughts and reflections into entire harmony with itself.
This is, philosophically, one of the first conditions of union with God. It seems to be naturally impossible, that we should realize an entire harmony or oneness with the divine mind, while the soul is so occupied with worldly thoughts flowing into it, as almost to shut out the very idea of God. A state of religious or spiritual quietude is, in other words, a state of rest in God. The idea of God, therefore, that magnificent and glorious idea, must so occupy the intellect, must be so interwoven with all its operations and modes of thinking, that the thoughts of other things, which so often agitate and afflict the religious mind, may be easily shut out.
And in order to do this, they, who would be perfect in Christ Jesus, must not mingle too much in the concerns of the world. Little have they to do with the unprofitable frivolities and pleasures of secular society; with idle village gossiping; with the trades and adventures and speculations of those who hasten to be rich; with the heats and recriminations of party politics, and many other things, which it would be easy to mention. No reading, also, should be indulged in, which shall tend to separate between the soul and God. Knowledge is profitable, it is true, but not all kinds of knowledge. It is better, certainly, if we cannot consistently with religious principles have a knowledge of both, to be familiar with the psalms of David, than with the poems of Homer; not only because the former are in a higher strain, but especially because heavenly inspiration should ever take precedence of that which is earthly. When, however, we read in the world's books from the sense of duty, when we may be said to read and study for God and with God, then, indeed, the great idea of the Divinity remains present and operative in the soul. And such inquiries and studies are always consistent with Christian quietude, because the mind, venturing forth at the requisition of the great Master within, returns instinctively at the appointed time, to the inward center of rest.
Hence we should lay it down as an important rule, to chasten the principle of curiosity, and to know nothing which cannot be made, either directly or indirectly, religiously profitable. Such knowledge, and such only, will harmonize with the presence of the great idea of God. All other knowledge tends to exclude it. And hence it is, that it can be so often said of those, who possess all worldly knowledge, to whom all arts and languages and sciences are familiar, that God is not in all their thoughts. The intellect is not in sufficient repose from the outward and purely worldly pressure constantly made upon it to receive Him. He comes to the door, but finds no entrance, and leaves them alone in their folly.
Perhaps in order to prevent mistakes, it should be added, that, when the mind is thus in a state of quietness and repose from worldly and errant imaginations, it does not by any means follow, as some may suppose, that it is, therefore, in a state of sluggish and insentient idleness. Not at all. No sooner has it reached the state of true stillness, by ceasing from its own imaginative vanities, and thus giving entrance to the purifying and absorbing conception of the great Divinity, than it becomes silently, but actively meditative on the great idea. Not, indeed, in a discursive and examinative way; not in a way of curious inquiry and of minute analysis; but still active and meditative. Much in the manner perhaps, that an affectionate child silently and delightedly meditates on the idea of an absent parent, not analytically and curiously, but with that high and beautiful meditation, which exists in connection with the purest love. Or much as any persons, who sustain to each other the relation of dear and intimate friendship, when in the providence of God they are separated at a distance, often repose in mental stillness from all other thoughts inconsistent with the one loved idea; and thus reciprocally the mind, active in respect to the object before it, though still and quiet in respect to every thing else, centers and dwell with each other's image.
—edited from The Interior or Hidden Life (2nd edition, 1844) Part 3, Chapter 10 by Thomas Cogswell Upham. His blog is managed by Craig L Adams and can be found here: http://thomascupham.blogspot.com