Crusade is a white enchantment card initially printed in Alpha. Its ability to "boost" white creatures by +1/+1 has ensured its play in white-heavy decks, of which include the famous archetype "White Weenie".
The optimal timing of when to cast this card is a very complex matter despite the simple function. Players using this card constantly have to weigh the amount of damage being dealt over time against the damage dealt after a crusade is put into play.
The artwork of Crusade was changed due to public outcry, similar to the case with Unholy Strength.