Scripture speaks about Christendom very simply and in only one way….
The first way, according to Scripture, is that Christendom means an assembly of all the people on earth who believe in Christ, as we pray in the Creed, “I believe in the Holy Spirit, the communion of saints.” This community or assembly means all those who live in true faith, hope, and love. Thus the essence, life, and nature of Christendom is not a physical assembly, but an assembly of hearts in one faith, as St. Paul says in Ephesians 4[:5], “One baptism, one faith, one Lord.” Accordingly, regardless of whether a thousand miles separates them physically, they are still called one assembly in spirit, as long as each one preaches, believes, hopes, loves, and lives like the other. So we sing about the Holy Spirit, “You have brought many tongues together into the unity of faith.” This is what spiritual unity really means, on the basis of which men are called a “communion of saints.” This unity alone is sufficient to create Christendom, and without it, no unity—be it that of city, time, persons, work, or whatever else it may be—can create Christendom.” (Luther’s Works, v.39, p.65)