Has this ideal of Christendom gone away?
Christendom may be gone as a matter of public law, and perhaps in the consciousness of most believers, but it’s still here as a substantive reality. Obedience and loyalty form a hierarchy for Christians, with God at the top, the Church and secular connections farther down, and natural law helping to sort and order the pieces and hold together the ones that can be used. If something in our present life finds a place in that hierarchy, it’s part of Christendom.