“Its life depends on the degree to which it is inhabited by mystery, speaks to us of the unknown.”

Psychology and Religion: What Is the Heroic Individual?
The Denial of Death (1973)

The Epistle to the Romans (1918; 1921)
Context: The revelation in Jesus, just because it is the revelation of the righteousness of God is at the same time the strongest conceivable veiling and unknowableness of God. In Jesus, God really becomes a mystery, makes himself known as the unknown, speaks as the eternally Silent One.<!-- p. 73

Lettre sur les aveugles [Letter on the Blind] (1749)
Context: As to all the outward signs that awaken within us feelings of sympathy and compassion, the blind are only affected by crying; I suspect them in general of lacking humanity. What difference is there for a blind man, between a man who is urinating, and man who, without crying out, is bleeding? And we ourselves, do we not cease to commiserate, when the distance or the smallness of the objects in question produce the same effect on us as the lack of sight produces in the blind man? All our virtues depend on the faculty of the senses, and on the degree to which external things affect us. Thus I do not doubt that, except for the fear of punishment, many people would not feel any remorse for killing a man from a distance at which he appeared no larger than a swallow. No more, at any rate, than they would for slaughtering a cow up close. If we feel compassion for a horse that suffers, but if we squash an ant without any scruple, isn’t the same principle at work?

A Night in May
The Venetian Bracelet (1829)

Source: Morals and Dogma of the Ancient and Accepted Scottish Rite of Freemasonry (1871), Ch. I : Apprentice, The Twelve-Inch Rule and Common Gavel, p. 1
Context: Though Masonry neither usurps the place of, nor apes religion, prayer is an essential part of our ceremonies. It is the aspiration of the soul toward the Absolute and Infinite Intelligence, which is the One Supreme Deity, most feebly and misunderstandingly characterized as an "architect." Certain faculties of man are directed toward the Unknown — thought, meditation, prayer. The unknown is an ocean, of which conscience is the compass. Thought, meditation, prayer, are the great mysterious pointings of the needle. It is a spiritual magnetism that thus connects the human soul with the Deity. These majestic irradiations of the soul pierce through the shadow toward the light.
It is but a shallow scoff to say that prayer is absurd, because it is not possible for us, by means of it, to persuade God to change His plans. He produces foreknown and foreintended effects, by the instrumentality of the forces of nature, all of which are His forces. Our own are part of these. Our free agency and our will are forces. We do not absurdly cease to make efforts to attain wealth or happiness, prolong life, and continue health, because we cannot by any effort change what is predestined. If the effort also is predestined, it is not the less our effort, made of our free will.

trans. Jasper Hopkins (Minneapolis: Arthur J Banning Press, 1990), 119 – 20.
De Docta Ignorantia (On Learned Ignorance) (1440)

" The Life and Teachings of Thoth Hermes Trismegistus http://magdelene.net/Thoth%20Hermes%20Trismegistus.htm", in The Secret Teachings of All Ages (1928) by the Canadian occultist Manly Hall; a few quotation websites credit this to Addison.
Misattributed

McKenna interview (1992)
Context: There's always fear of the unknown where there's mystery. It's possible to achieve a state where you realize the truth of life and fear disappears, and a lot of people have reached that state, but next to none of them are on Earth. There's probably a few.