Christendom
Part of speech: proper noun
Definition: The Christian world.
Christianity
Part of speech: noun
Definition: A monotheistic religion based on the teachings of Jesus Christ and various scholars who wrote the Christian bible.
Example sentence: Christianity will go. It will vanish and shrink. I needn't argue with that; I'm right and I will be proved right. We're more popular than Jesus now; I don't know which will go first - rock and roll or Christianity.