By Robert Pigott
BBC religious affairs correspondent
Russian President Vladimir Putin is about to pay his first visit to the Vatican of Pope Benedict XVI.
President Putin met John Paul II - but never invited him to Moscow
He wants the Pope on his side as he restores Russia as a global force.
It will be Mr Putin's third visit to the Vatican and his second pope. He went to see Pope John Paul II in 2000 and 2003, coming face-to-face with a man who had contributed to the dissolution of the Soviet bloc.
But he was one of the few heads of state who did not attend John Paul II's funeral. That absence is perhaps partly explained by the long deep-freeze in relations between the Vatican and the Russian Orthodox Church, and the remarkable church-state alliance that has sprung up in Russia.
The Moscow branch of Orthodoxy has often accused the Vatican of seeking to poach its members in Russia.
John Paul II reinforced the Roman Catholic Church's structure in Russia, despite the evident damage to his heartfelt wish to visit Moscow.
At the same time, the Russian Orthodox Church was being recruited by President Putin to fill the gap left by Communism.
The powerful Pope is not a man Russia wants as an enemy
The Church is patriotic to the point of being nationalistic - a useful organising principle for a society suddenly cast adrift, and a valuable source of moral support for the Putin government.
In return it has had financial support and, some argue, state intervention to protect its leading role in society against potential competitors - including the Roman Catholic Church.
Perhaps that is why President Putin did not - unlike his predecessors Mikhail Gorbachev and Boris Yeltsin - invite John Paul II to Russia.
But there has been a noticeable thaw in relations since Pope Benedict's election in April 2005.
Senior clergy from the Vatican attended an inter-faith summit in Moscow last July, and an Orthodox official is thought likely to be among the Russian delegation with President Putin on Tuesday.
In 2005 President Putin visited the Orthodox monks at Mt Athos
The president - himself baptised as a Russian Orthodox - has said he can mediate between the two churches.
There are other hopeful signs. Russian Orthodox Patriarch Alexy II welcomed what he called the Pope's "willingness to develop ties with the Orthodox Church".
But there remains plenty to do.
Mr Gorbachev, just after a visit to the Vatican, granted freedom of operation to all religious groups in 1990, when the Soviet Union still existed.
However, seven years later, a new law limited that freedom to what were described as "traditional" religions. "Non-traditional" religions were impeded or blocked altogether, and the Roman Catholic Church was among them.
Relations reached a new low in 2002, when Roman Catholic priests were refused renewal for their visas, despite working in Russia for many years.
Scope for change
There are signs that the special relationship between the Russian Orthodox Church and state will prevent a significant relaxation in the Vatican's freedom of movement there.
Pope Benedict has shown himself less of a traveller than John Paul II, but - judging by his efforts to mend troubled relations with the Greek Orthodox Church - he would probably prize an invitation to Moscow highly.
Since Soviet dictator Joseph Stalin sarcastically asked "How many divisions does the Pope have?" Russian leaders have come to realise the moral and "soft" political power that resides in the Vatican.
It is a tribute to the continuing influence of a Church that claims more than a billion members that President Putin wishes to visit Pope Benedict.
President Putin has ambitions for his country as a global power. Russia will encounter many influential Roman Catholics in the globalised world, and will not want to have made an enemy of their spiritual leader.