How did a knight become a crusader?

a knight became a crusader when he rode with the armies to the holy land

A crusade in Medeval Catholic Europe was called by the pope. If a knight took up the call to arms and travelled to the destination (the most famous being of course Jerusalem) he became a crusader knight. However any man who travelled to the holy land on request of the pope was a crusader, even peasants. people often went on crusade for the promise of pludering the rich Islamic States but also because it was told that any man who died on cursade would have all his sins forgiven and gain acess to heaven