By James Rodgers
BBC News, Moscow
Alexiy II enjoyed close relations with the Kremlin
When Alexiy II became head of the Russian Orthodox Church, it was forced to co-exist with a state which was officially atheist.
Now, even though only a minority of Russians actually attend services, opinion polls suggest that a clear majority of the people readily identify themselves as Orthodox Christians.
Under Alexiy II's leadership, the Russian Orthodox Church strove to revive traditional values, to fill the ideological and spiritual vacuum which followed the collapse of Communism.
For that reason, he resisted Roman Catholic attempts to increase their church's influence in the former Soviet Union, blocking plans for the then Pope, John Paul II, to visit Russia.
After the election of Pope Benedict in 2005, there had been talks between Orthodox and Catholic clergy about a possible meeting between the two leaders, but it never came to pass.
Not only was he a prominent figure in the history of the Russian Orthodox Church, but also a great statesman
Russian Prime Minister
As he worked to restore Orthodoxy's spiritual strength in Russia, Alexiy II enjoyed an ever higher public profile.
His activities were often reported on state television, in a country where such coverage is often reserved for those who wield great political power.
As president, Vladimir Putin was frequently seen at church services on major religious festivals - pictured at the patriarch's side.
In paying tribute to Alexiy II, Mr Putin described a man whose authority extended beyond the Church.
"The death of the patriarch is really a very tragic and sorrowful event. He was a very serene man. It is a great loss," the Russian prime minister said.
"Not only was he a prominent figure in the history of the Russian Orthodox Church, but also a great statesman."
One of Alexiy II's greatest achievements was the reunification in 2007 of the Church in Russia, and abroad. The split had existed since exiles who fled the Bolshevik revolution in 1917 set up their own church overseas.
Mr Putin's successor as president, Dmitry Medvedev, said of the patriarch: "Not only was he an outstanding religious figure and true spiritual leader, he was also a great citizen of Russia."
Mikhail Gorbachev, the last leader of the Soviet Union, has expressed great personal sadness.
"I am so shocked that it is very hard to me to find words on the spot," he told Interfax news agency. "I respected him deeply."
The Church's increasing importance in society did not please everyone.
Last year, a group of members of the Russian Academy of Sciences wrote an open letter to Mr Putin, expressing concern at what they called the Church's growing influence in non-religious circles.
As head of the Church, Alexiy II presided over a major programme of church restoration and rebuilding. Hundreds of churches from the remotest villages to the capital were restored and reopened during his time as patriarch.
His successor is expected to be elected by an assembly of church members and bishops which must be convened within six months.
The Kremlin has no formal say in the choice, but, because of the influence which comes with the office, the political authorities are likely to watch the process very closely.