The word Christendom generally refers to the global community of those who adhere to the Christian faith, with religious practices and dogmas gleaned from the teachings of the Bible. Spread throughout the world, Christendom consists of literally billions of people among many nations and peoples of various ethnicities. Correspondingly, Christendom also refers to those countries where Christianity is the dominating or territorial religion.
Whereas Christendom reflects the community, meaning its bodies of people who are guided or governed by Christian institutions and its authorities such as the church, Christianity as a religion is more about a relationship than religious practices. Whereas other religions of the world teach that one must do or perform certain tasks or works in order to appease their god or gods, Christianity teaches that we are made right before God through our faith in Jesus Christ and what He did on the cross on our behalf.
Not all those who live under the general term “Christendom” are adherents of the Christian faith. The once-Christian European nations are still technically under what is known as “Christendom,” but, for the most part, biblical Christianity has been set aside in favor of secular humanism.