Patriarch Kirill, enthroned on Sunday as the new leader of around 160 million Russian Orthodox believers, is seen as an outspoken moderniser who may thaw icy ties with the Catholic Church.
The first Patriarch elected since the collapse of the Soviet Union in 1991, Patriarch Kirill, 62, replaces Alexiy II, a conservative credited with reviving the Church and transforming its place in Russian society before his death from illness last month.
His tenure as the 16th Russian Orthodox Patriarch, like that of Partiarch Alexiy before him, is likely to be defined by his relations to two powerful institutions: The Kremlin and the Catholic Church.
Some hope Patriarch Kirill will establish better ties with Catholics than his predecessor, who accused Rome of trying to poach Orthodox believers and resisted meeting the pope.
Hopes of a thaw have been fuelled by Patriatch Kirill's meeting with Pope Benedict in the Vatican in 2007 and his optimistic comments about better relations with Rome.
But he has also echoed Patriarch Alexei's criticisms of Catholics on occasions. "He is probably more sympathetic to improving relations with the Vatican," said Professor John Anderson, an expert on Russian Orthodoxy at the University of St Andrews in Scotland.
"He also has to watch what he says" considering how conservative the rest of the Church is, he added.
In an interview given shortly before his election as patriarch, he said a meeting with the Pope would be possible "when there are conclusive signs of real and positive progress" on issues dividing the Churches.
Like Rome, the Kremlin will loom large over Patriarch Kirill's tenure.
Patriarch Alexiy's legacy of pushing the Church back to the centre of Russian life after the fall of atheist communism in Russia was largely based on close ties with political leaders, who helped fund a huge wave of church building.
Church officials are given pride of place at state occasions and political leaders are regularly filmed attending mass on major Church holidays.
In a 2005 letter to then US Secretary of State Condoleezza Rice, Patriarch Kirill described the Russian Orthodox Church as "completely separate from the state apparatus" and said clerics would never involve themselves in politics.
But both Prime Minister Vladimir Putin and President Dmitry Medvedev stood at the side of the alter on Sunday when he was enthroned as Patriarch. Mr Medvedev told worshippers he hoped Patriarch Kirill would help foster dialogue between Church and state.
Canon Michael Bordeaux, head of the Keston Institute that monitors religion in former communist states, said close ties with the Kremlin are unlikely to weaken any time soon.
"As it is, the Moscow Patriarchate never criticises the Kremlin over its internal or external policies... I don't think Patriarch Kirill will start doing that."
"In the Orthodox tradition, which goes back 500 years, the state and the Church work together," he said.
Born in Leningrad, now called St Petersburg, into a priest's family, Patriarch Kirill was ordained a priest in 1969. He later served as rector of the Leningrad seminary, regarded as one of the most open to the West.