As usual, as in so many feel-good (feel-well?) sayings, some context has been elided. What she said was, "It behoved that there should be sin; but all shall be well, and all shall be well, and all manner of thing shall be well." That is, the world is a dark place, but it turns out well.
The good Dame, however, said many other things that make me (personally) feel better. One such comes in her Sixth Revelation, when she says, "The age of every man shall be acknowledged before him in Heaven, and every man shall be rewarded for his willing service and for his time." I would like to believe this is true, in my more self-pitying moments. Heh.
To get there however, one needs some knowledge of her Second Revelation, in which she says, "It is God’s will that we have three things in our seeking: — The first is that we seek earnestly and diligently, without sloth, and, as it may be through His grace, without unreasonable heaviness and vain sorrow. The second is, that we abide Him steadfastly for His love, without murmuring and striving against Him, to our life’s end: for it shall last but awhile. The third is that we trust in Him mightily of full assured faith."
But the mystery of belief is still there in the Thirteenth. I can't stop thinking about the subtleties of this line ascribed by the Dame to her Master: "I may make all thing well, I can make all thing well, I will make all thing well, and I shall make all thing well; and thou shalt see thyself that all manner of thing shall be well."
Maybe I'll post on this again.