In Christendom there are groups who find themselves exclusive, believing they are the holders of the absolute truth, while all others are heretics or even the work of the devil himself.
From those preaching the Gospel and Baptism in Jesus name
Always there have been real followers of Jesus who only worshipped his One God, the God of Israel. Many centuries they worked and preached behind closed doors, avoiding persecution and death on the balefire. In fear of the stake for centuries the real followers of God baptised people in their faith, not demanding to be part of one or another church but being part of the Body of Christ.
Christian in Christendom or in Christianity
Often a lot of people do not know to differentiate between Christendom, Christianity, religiosity and even limit religiousness to Christians, forgetting that there are many other religious groups who have all sorts of worship.