During the Middle Ages, Europeans sent a series of military expeditions through the Holy Land in an attempt to win parts of it from Muslim powers.
These self-proclaimed "soldiers of the Church" carried crosses (in Latin, crux), which gave the campaigns their name: Crusades.
Since then, the word has mellowed considerably into its current meaning: "an effort to change something, carried out with zeal and enthusiasm."
For example, as Time reported, "The International Table Tennis Federation is on a crusade to make table tennis more fan and viewer friendly." (Alice Park, Aug. 21, 2008)