Five hundred years ago the Reformation ripped a major part of Christianity apart when Martin Luther led protests practices by the Church that reverberated for centuries.
But 500 years before that the Great Schism of 1054, when the Greek Orthodox Church officially split with the Catholic Church had also split Christianity.
Today Orthodox Christians are concentrated in Europe, where they have declined as a percentage of the global population, but its Ethiopian community is highly observant and growing.
The largest Orthodox Christian population outside of Eastern Europe is today in Ethiopia.
The centuries-old Ethiopian Orthodox Tewahedo Church has an estimated 36 million adherents, nearly 14 percent of the world's total Orthodox population. This East African outpost of Orthodoxy reflects two broad trends.
First, its Orthodox Christian population has grown much faster than Europe's over the past 100 years. And, second, Orthodox Christians in Ethiopia are more religiously observant, by several common measures, than Orthodox Christians in Europe.
This is in line with a broader pattern in which Europeans are, on average, less religiously committed than people in Latin America and sub-Saharan Africa, a new Pew Research Center surveys show.
Pew notes that this is true not just of Christians in Europe but also of Europe's Muslims, who are less religiously observant, as a whole, thab Muslims elsewhere in the world.
Over the last century, the Orthodox Christian population around the world has more than doubled and now stands at nearly 260 million, a Pew Research Center survey on Orthodox Christianity in the 21st century shows.
THIRD LARGEST BRANCH OF CHRISTIANITY
Orthodoxy is the third-largest branch of Christianity, after Catholicism and Protestantism.
Orthodoxy, or Eastern Christianity, formally split from Roman Catholicism (known then as Western Christianity) in 1054 over a host of theological issues, high among them disputes over papal authority.
Today in Russia alone, it has surpassed 100 million, a sharp resurgence after the fall of the Soviet Union when Christianity was suppressed.
Yet despite these increases in absolute numbers, Orthodox Christians have been declining as a share of the overall Christian population – and the global population – due to far faster growth among Protestants, Catholics and non-Christians.
Today, just 12 percent of Christians around the world are Orthodox, compared with an estimated 20 percent a century ago. And 4 percent of the total global population is Orthodox, compared with an estimated 7 percent in 1910.
And a century ago, all three major branches of Christianity were concentrated in Europe. That is still the case for Orthodox Christians, but not Protestants and Catholics.
The Pew survey shows that the geographic distribution of Orthodoxy also differs from the other major Christian traditions in the 21st century.
In 1910 – shortly before World War I, the Bolshevik revolution in Russia and the breakup of several European empires – all three major branches of Christianity (Orthodoxy, Catholicism and Protestantism) were predominantly concentrated in Europe.
Since then, Catholics and Protestants have expanded enormously outside the continent, while Orthodoxy remains largely centered in Europe.
Today, nearly four-in-five Orthodox Christians (77 percent) live in Europe, a relatively modest change from a century ago (91 percent).
By contrast, only about one-quarter of Catholics (24 percent) and one-in-eight Protestants (12 percent) now live in Europe, down from an estimated 65 percent, and 52 percent, respectively, in 1910.
The survey connects Orthodoxy's falling share of the global Christian population to demographic trends in Europe, which has lower overall fertility rates and an older population than developing regions of the world, such as sub-Saharan Africa, Latin America and South Asia.