Here’s a point made by Christian apologists that comes out in spades this time of year—that, regardless of belief or religion, the whole world dates their calendar from the birth of Jesus. This is (mostly) true, but pointing it out does not make the point many Christians seem to think it makes. Indeed, far from something to boast about, the fact that (practically) the entire world uses the Christian calendar is something that ought to make Christians somewhat reticent—because of the way it came about.
Christianity became the Roman Empire’s state religion in 380 AD. When Rome fell a century later Christianity survived, and, by means of various institutional structures, spread to all of Europe in the centuries that followed. In the late 8th century, Christian scholars in Europe recommended dating the current year from the year (they believed) Jesus was born. The Emperor Charlemagne endorsed the practice, but it was not until 1422 that it was adopted in all of western Europe.
Then, in the fifteenth through mid-twentieth centuries, the Christian nations of Europe colonized nearly the entire world.
So yes, in the year 2017, (close to) the entire world uses a calendar that dates from the year Jesus was born (give or take a few years). That does not, however, say anything about the strength of Christian ideas and beliefs—unless you want to argue that it was Christian ideas and beliefs that enabled the Europeans to (with much bloodshed, pain, and suffering) colonize nearly the entire world.
I have sometimes seen a form of this argument made—that it was Christian (or, sometimes, Protestant) ideas that enabled the countries of western Europe to grow powerful and successful. The trouble with this argument is that, regardless of the veracity of its claim, centuries of bloody military conquest, control, and colonization, with all of the abuses that came with it, fits awkwardly over Christian claims to be a religion of grace and compassion, or a religion that spreads on primacy of ideas and not through violent conquest.
When a Christian apologist declares that the whole world uses a calendar that begins with the birth of Jesus, as though they are making a point, what they are really saying is that Christianity was state religion of the handful of nations of the past few centuries most efficient at waging often brutal military conquest.* For a religion that is ostensibly supposed to spread by voluntary conversion—a religion based on belief and not outward practice—that seems a very a odd thing to boast about.
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