Christendom, however, is something quite different from Christianity, being the administrative or power structure, based on the Christian religion and constructed by men. It bears the same relation to the everlasting truth of the Christian revelation as, say, laws do to justice, or morality to goodness, or carnality to love—if you like, as Augustine’s City of God to the earthly city where we temporarily live.
The founder of Christianity was, of course, Christ. The founder of Christendom I suppose could be named as the Emperor Constantine. I believe and Bishop Sheen believes that it is not Christ’s Christianity which is now floundering. You might even say that Christ himself abolished Christendom before it began by stating that his kingdom was not of this world—one of the most far reaching and important of all his statements. Christendom began with the Emperor Constantine. Christianity began with the Incarnation.
— Malcolm Muggeridge, from The End of Christendom, p. 13