Christians today sometimes hear and use the word Christendom, almost always, it seems, in a negative light. Christendom was a time (so the narrative goes) when Christian rulers persecuted unbelievers, when church and state were unbreakably united, when slavery was rampant, and when moralist hypocrisy was everywhere. Secularists from the French Revolution down to today as well as Anabaptist Brian McLaren and the Reformed theologian David VanDrunen are all enemies of Christendom. The contra-Christendom cadre is diverse indeed.
A more accurate and less tendentious description was that it a time in Western civilization from roughly the 5th to the 18th centuries when the Christian Faith dominated all of life, not just the church. Architecture and art and education and law and music and politics and science were all rooted in what we nowadays term a Christian worldview. Society itself was justified by appeal to the Christian Faith. Nations waged war (rightly or wrongly) in the name of Jesus Christ. The Christian Faith was not a religion you adopted as one among many “options.” You were born and baptized into, lived and loved and worked and played and died within a Christian society. Your life was enveloped in a Christian milieu.