The mills of God grind slow...

"The mills of God grind slowly, yet they grind exceeding small" (sometimes quoted as "The mills of God grind slow, but they grind exceeding small") is a well known English proverb.

Its meaning is that divine retribution is slow but certain.  

Its origins are less well known. I had always assumed it arose from The Bible or some early holy text.  But it comes from "Retribution", a short poem by Henry Wadsworth Longfellow (1807-1882):

“Though the mills of God grind slowly;

Yet they grind exceeding small;

Though with patience he stands waiting,

With exactness grinds he all.”

Longfellow derived this from the poetical works of 17^th century German poet, Friedrich von Logau.  

 Von Logau, in turn, was translating a hexameter from Adversus Mathematicos (Against the Mathematicians) by a Greek sceptic physician and philosopher, Sextus Empiricus (c. 160-210 AD). 

As if that were not labyrinthine enough, Sextus Empiricus was himself quoting an unknown poet! So we haven't a clue where it first came from!