You might find the French quotation, as I did on Anzac Day in Amiens near the cathedral, on a statue. Though it was originally pronounced by Pope Urban II in 1095 as he launched the First Crusade under the leadership of Godefroy de Bouillon, later King of Jerusalem, whose massacres sanctioned by the Church and rewarded by plenary indulgences and forgiveness of his and his murderous knights (other) sins, it has since been used to justify all sorts of crimes against humanity, including those of the “Great” War which I had come to the Somme to commemorate.
In the Middle Ages, it became the motto of the Order of the Holy Sepulcher. It was originally proclaimed in Latin (Deus vult, become Deus lo vult in low Latin) and more recently as “God wills it” or “Gott will es”. (The Inch’Allah of Muslims is a conditional (“God willing”) or optative (“Please God”) form, rather than the dogmatic certitude expressed by their crusading enemies of yore and of Christian believers today.)
It is, of course, an extraordinary feat of blind arrogance to presume to know the will of God, even if He did exist. Mind you, in the realm of the blind folly that is blind faith, why not ? “God acts in mysterious ways”, “God will provide”, “In God we trust” — believers believe in God, proclaim their trust in Him, confident that they know exactly what He wants, especially if it is what they want themselves.
The conviction has its advantages, especially when there is question of getting people to accept a difficult, dangerous or dubious course of action. The Crusades are the classic example. The American invasion of Irak is a more recent one. God only knows what it will be used to justify next. If it made any sense, one could say, in desperation, “God help us !”