Christianity - Christendom
From Hull AWE
Be careful not to confuse Christianity and Christendom.
Christianity is the set of beliefs and practices which constitute the Christian religion.
Christendom is the entire body of Christians throughout the world and/or throughout history. Thus when it was said of King James I of England (and VI of Scotland) that he was 'the wisest fool in Christendom' (because 'he never said a foolish thing and never did a wise one'), what was meant was that King James was the 'wisest fool' amongst all the Christians in the world.
