This phrase originates in the First Epistle to the Corinthians; chapter 13, verse 12, of the generally accepted order. I have irritated several of the people on this site, and many more outside of it, by insisting that, even taken solely as a work of literature, the Bible is one of the best, if not the best, works of prose in the world; and this is one of its very finest pieces. The immediate context is as follows (in the King James translation, a remarkable achievement in its own right):
...whither there be prophecies, they shall fail; whither there be tongues, they shall cease; whither there be knowledge, it shall vanish away. For we know in part, and we prophesy in part. But when that which is perfect is come, then that which is in part shall be done away. When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
The rest of this chapter is a pæan to the greatness of charity, often translated in later English editions as love — which is inferior for various reasons which we won't go into now; but here the text breaks off and becomes something rather different; a strange, nearly ominous prophecy. This parallels the older moment, another of the Bible's best parts, in the Book of Job, where God speaks to Job out of the whirlwind, and instead of giving any answers, replies to Job's questions with a barrage of questions of His own; questions which go to prove only that Job knows little or nothing; that he knows, as Paul has it, in part. But one of these questions is more than just dismissive:
Where wast thou when [...] all the sons of God shouted for joy?
This does tell us something, something startling and profound: that, despite Job's suffering and doubt, when the sons of God beheld the world in the morning light of creation, it was a joy unto them — and why this should be, the origin of this other joy, is clearly something that Job knows nothing about.
That is: there is a great secret. We do not know this secret: but we will know it. In the Old Testament, that last part, the promise, is left implied, a groping thing; but in the New Testament, Paul says it outright. We, too, are the sons of God: and one day we will shout for joy.
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I am not the only one whom this passage has struck with singular power; on the contrary, it seems to have been equally effective these whole two thousand years. Any number of books are named after it, or make reference to it (as for instance Horselover Fat's A Scanner Darkly); uncountable sermons, speeches and addresses have made use of it. It is a treasure in Greek, in Latin, and also in English. It is good for everything, and every occasion, from the physicist who intones it on discovering a new particle, to the schizophrenic muttering it while taping an incoherent screed to an outdoor fuse-box.