The formerly rival churches will seal their historic reconciliation at a ceremony on May 17 in the Russian capital's largest cathedral.
President Vladimir Putin has hailed the move, which will end more than eight decades of bitter estrangement, as an "epoch-making event."
The head of the Russian Orthodox Church, Patriarch Aleksy II, and Metropolitan Lavr, the leader of the New York-based Church Abroad, are due to sign an "act of restoration of canonical relations."
Clergymen from both churches will then celebrate a lavish joint Mass in Moscow's Christ the Savior Cathedral. Moscow's largest cathedral was blown up by Soviet dictator Josef Stalin and rebuilt after the 1991 Soviet collapse.
End To A 'Difficult History'
Metropolitan Kirill of Smolensk and Kaliningrad, the head of external relations for the Moscow Patriarchate, expressed hope that the event will heal a painful rift.
"This is a historic event that we hope, by God's grace and the prayers of Russia's New Martyrs [repressed under Soviet rule], will put an end to the long and difficult history of the division of our people," he said.
Metropolitan Lavr (pictured right) was equally upbeat as he arrived in Moscow on May 15 with dozens of priests and choristers.
"I think it's a very important event, of course, and it is particularly important for the Russian Church, which has been divided for 90 years," he said.
The reunification is an important step for Russia in coming to terms with its communist past.
Founded by clerics who fled the 1917 Bolshevik Revolution, the breakaway Orthodox church was first based in Stavropol, a southern Russian city then controlled by the White Army.
As the Red Army advanced, the church moved to Ottoman Turkey, and then to Serbia, before establishing its headquarters in New York.
Church 'Stained' By Soviet Past
The Church Abroad severed all ties with Moscow in 1927 after the Moscow Patriarchate signed a declaration of loyalty to the atheist Soviet regime.
Contacts were only officially renewed in 2003, when the Moscow Patriarchate recanted the declaration as a "tragic compromise."
Some are worried the Moscow Patriarchate is focused on "getting its hands on the property of the Church Abroad." -- Xenia Dennen of the Keston Institute
Not all members of the exiled church, however, welcome the reunification.
Some say the Moscow Patriarchate is stained by its Soviet past and condemn its ties with President Vladimir Putin, a former Soviet intelligence officer.
Xenia Dennen, the head of the Keston Institute, a London-based group that studies religious issues in communist and former communist countries, says the reunification is significant.
"It will mean a great deal for those who couldn't receive communion from the Russian Orthodox Church Abroad," she says. "However, there's a lot of disagreement within the Russian Orthodox Church Abroad. There are fears that the Moscow Patriarchate's focus is particularly on power control, on getting its hands on the property of the Russian Orthodox Church Abroad."
Moscow's Christ the Savior Cathedral (ITAR-TASS)
Under the new arrangement, the Church Abroad, which claims 500,000 members in more than 30 countries, will retain control over its property and continue to appoint its own priests.
The Moscow Patriarchate will gain the right to approve the nomination of new leaders of the Church Abroad. Parishes of the Church Abroad will also now mention the patriarch's name during Mass.
Good For Putin's Image
Both supporters and critics of the reunification agree that the event's significance extends far beyond religion.
"For the regime, this is of course a big advertisement project," says Mark Smirnov, editor in chief of the religion magazine of the Russian daily "Nezavisimaya gazeta." "Because it enables Putin to show that during his tenure as president he was not only a rather successful manager, but also the one who united the churches. One can see it as a historic step that will figure in large print in Vladimir Putin's biography."
Dennen of the Keston Institute views the reunification as part of a broader Kremlin campaign to buff up Russia's image.
"Of course the Orthodox Church is very grand and very beautiful, and that [reunification] will contribute to that image," she says. "I think, although legally it isn't the established church, it in fact it behaves as though it were, and Putin's regime certainly makes good use of that."
Putin, here being blessed by Aleksy II in 2006, is an Orthodox believer (epa)
Putin has strongly supported the reconciliation between the two churches, as well as other steps to revive Russia's pre revolutionary past. While some have welcomed his efforts, others have accused him of stirring nationalist feelings.
The Russian Orthodox Church in 2000 canonized Russia's last tsar, Nikolai II, and his family, who were murdered by the Bolsheviks.
Last year, the remains of the tsar's mother, Empress Maria Fyodorovna, who died in exile in Denmark, were reburied in St. Petersburg.
Putin is expected to attend the May 17 ceremony together with the descendents of the Romanov dynasty.