It is pretty much accepted that the world changed around 1917; but why it changed has several explanations.
1. An economic explanation: The 1914-18 Great War: the exhaustion of European powers, massive destruction, trade disruption...
2. A psychological explanation: the trauma of the above war, the slaughter of the flower of young men, end of idealism...
3. A socio-political explanation: The Russian Revolution, specifically the Bolshevik takeover: world communism, the first of many atheist totalitarian tyrannies...
4. A Christian explanation: The martyrdom of Tsar and Saint Nicholas II:
That which restraineth the appearance in the world of the Antichrist, the man of lawlessness and anarchy, the last and most powerful enemy of Christ and His Church, is – in the teaching of St. John Chrysostom and others Fathers of the Church – lawful authority, as represented and symbolized by the Roman Empire. This idea was incarnated supremely in the Christian Empire: first in Byzantium, when Constantinople was the Second Rome, and then in the Orthodox Russian Empire, when Moscow was the Third Rome. In 1917 the “Constantinian Age” came to an end, the Orthodox Empire was overthrown – and the world, beginning with Moscow, has been thrown into an age of lawlessness and atheism (and in Christian life, of apostasy) such as has not yet been seen.
Fr Seraphim Rose