'Christendom' originates from Old English, specifically the word 'cristendom,' where 'cristen' meant 'Christian' and '-dom' meant 'domain or realm.'

'Cristendom' changed from the Old English word 'cristendom' and eventually became the modern English word 'Christendom'.

Initially, it meant 'the domain or realm of Christians,' and this meaning has largely remained the same in modern usage.