As you might expect, the decline began in pre-revolutionary Russia. The parallels between that situation, and that of Canada (especially Quebec) are quite interesting.
This makes me think of (the spirit of) Vatican II:
I remember reading a book by Father Georgy Shavelsky, the Protopresbyter of the Russian Army and Navy under Nicholas II. Himself one of the senior members of the Holy Synod, he testified that the Synod was in fact very far from the life of people, that it did very little (if anything) to prevent atheist propaganda from spreading among ordinary people. To show how little remained of the people's traditional devotion to God, Shavelsky cites the following example: when attendance at the Liturgy became, by a special imperial decree, no longer obligatory for Russian soldiers, only ten percent of them continued to go to church.
Another testimony of the same kind is that of Metropolitan Veniamin (Fedchenkov), who became the Bishop of the White Army after the revolution. He writes that none of the students of the St. Petersburg Theological Academy, where he had studied, ever went to see Father John of Kronstadt, and that some of the students were atheists. He describes the atmosphere of spiritual coolness inside the Orthodox Church, the lack of prophetic spirit. He claims that it was not by mere chance that there arose people like Rasputin: against the common background of indifference towards religion he appeared as a charismatic figure and was at first accepted as such by the ecclesiastical authorities, who then directed his steps to the imperial palace.
The picture which one gains when reading the memoirs of those living during the pre-revolutionary period is that of a deep decline in religious belief. Though Orthodox Christianity was still maintained as the official religion of the Russian monarchy, both society and the Church were fatally contaminated by unbelief, nihilism and atheism. Even the seminarists, future priests, balanced on the edge between religion and atheism. Many ordinary Christians, if not the majority, had no faith at all, and it was they who turned against the Church as soon as membership in it stopped being encouraged. The Church at once lost the great majority of its members and remained a small flock of those prepared to die for Christ.
H/T: Walk with Men