By Wikipedia:

The Protestant work ethic, sometimes called the Puritan work ethic, is a Calvinist value emphasizing the necessity of constant labor in a person's calling as a sign of personal salvation. Protestants beginning with Martin Luther had reconceptualised work as a duty in the world for the benefit of the individual and society as a whole. The Catholic idea of good works was transformed into an obligation to work diligently as a sign of grace.

http://en.wikipedia.org/wiki/Protestant_work_ethic