The term Deus vult is Latin for “God wills it.” It was a very popular rallying cry at the time of the First Crusade in the 11th century, and some members of certain Christian sects continue to use it, although with slightly less aggressive connotations. Although most people do not speak or study Latin in the modern era, the term is part of a library of Latin phrases that have endured because of their religious associations.
According to historians who wrote about the First Crusade, when Pope Urban II announced that European Christians were going to ride to the defense of Christians in the East, members of the crowd spontaneously started shouting “Deus vult,” suggesting that God willed the crusade. Given that many people spoke Vulgar Latin, rather than Classical Latin, it is far more likely that the crowds shouted “Deus lo vult,” in strict point of fact, although a few erudite priests may have stuck to the Classical Latin form.
People may see this phrase written as "Deus lo vult," most notably on the coat of arms of the Order of the Holy Sepulchre, a Catholic order of knighthood that dates back to the time of the First Crusade. Early members of the Order undoubtedly rode to battle with cries of the saying ringing in their ears, as has been clearly documented by historians who wrote about the First Crusade, so one can see why it had appeal as a motto.
While the term is closely linked with the violence of the Crusades for some people, it could also be viewed in a different way. In the 1996 science fiction novel The Sparrow, several of the Jesuit characters frequently exclaim “Deus vult” in response to something wonderful or astounding, using the term to remark on the marvel of God's works and will. Characters in the novel also say it to explain the unexplainable, using it to mean “God likes it this way.”
Many religions have some similar phrase in their vocabulary, using the term to comment on life events or to motivate people. On occasion, the will of God may seem contradictory or frustrating, so the reminder that the works of God do have a purpose, even if it seems unclear, can be very comforting for some people.