In the  face of the militant atheism of the Bolshevik revolution, the Russian Church split into two, with the flame of traditional Orthodoxy kept burning in exile while the Church at home fell under the control of the hammer and sickle. Now, nearly 90 years on, unity has been restored

With a stroke of his favourite green felt-tipped pen, Patriarch Alexis II of Moscow and All Russia has put an end to the Russian civil war - nearly 90 years since the Bolshevik revolution de facto started it. On 17 May in a splendid ceremony in Christ the Saviour's Cathedral, diocesan church of the Russian capital, the Russian Orthodox Church of the Moscow Patriarchate reunited with the Russian Orthodox Church Abroad, officially referred to as the Russian Orthodox Church Outside of Russia, or Rocor.

Russian émigré bishops created the latter in the 1920s, when it had become clear that whatever remained of the Orthodox Church in Russia would have to exist at the mercy and under tight control of a militantly atheist regime. But last month, Metropolitan Laurus, head of the Russian Orthodox Church Abroad, flew from his diocesan headquarters in the state of New York, joined Patriarch Alexis in a divine service after the unity ceremony, and took Communion from the same cup.

Only 10 years ago, Laurus' predecessor, Metropolitan Vitaly, insisted on upholding a decades-old anathema against what Rocor routinely referred to as the "KGB-controlled" Moscow Patriarchate.

Standing on the ambo of the Saviour's Cathedral, the only journalist among the priests and Religious, just a few metres from the table where the Act of Canonical Communion was to be signed, I listened to the reading of the historical document: "By this Act, Canonical Communion within the Local Russian Orthodox Church is hereby restored. Acts issued previously which preclude the fullness of Canonical Communion are hereby deemed invalid or obsolete."

I looked at Patriarch Alexis, his usual concentrated and composed self, and wondered whether he felt what everyone in the gigantic cathedral knew - at this very moment he goes down in history as one of Russia's greatest religious figures. He had presided over not only an unprecedented revival of Russia's Christianity after the fall of Communism, but also became the primate who finally made the wounds of civil war heal for Russia's oldest historical institution.

Through the years, the Russian Orthodox Church Abroad remained staunchly nationalist, anti-ecumenical and monarchist. In 1981, it proclaimed Emperor Nicholas II and his family, murdered by the Communists in 1918, saints. However, the Church that claimed to serve two million Russians in exile in the 1920s and 1930s gradually dwindled in numbers. Some say that today it serves no more than 100,000 believers and has around 250 churches. For hundreds of thousands of Russians from post-Communist Russia, who criss-cross the world today, not to be able to enter a Russian church and pray, not to mention taking Communion, seemed illogical, incomprehensible and unjust. In comparison, the Moscow Patriarchate parishes in the former Soviet republics number more than 27,000, with the number of believers in tens of millions, as well as nearly 30,000 priests and 16,000 monks and nuns.

Few in Moscow doubt that at some point the distinction between the Church Outside of Russia and the rest of the Russian Church will blur. "When in 1927 it severed all ties with the Church in Russia, [Rocor] proclaimed this a temporary condition, existing as long as religious life in Russia is not free," Fr Nikolai Blashov of the Moscow Patriarchate's external relations department told me. For now, Rocor will be granted full autonomy. However, as the Act says, "the name of the Primate of the Russian Orthodox Church and the name of the First Hierarch of the Russian Orthodox Church Outside of Russia are commemorated during divine services in all churches of the Russian Orthodox Church Outside of Russia before the name of the ruling bishop in the prescribed order." The head of Rocor will be elected by its Council of Bishops, and then confirmed by the Moscow Patriarch and the Holy Synod of the Russian Orthodox Church. To lay people this means that, in contrast to only a few days ago, they can partake in Communion in practically any Russian church in the world.

The Moscow Patriarchate sees reunification as an important step towards spreading its global influence. It is already the biggest in number of believers among the 15 autocephalous Orthodox Churches, while parishes of the former émigré Church exist on all five continents.

The Church Abroad possesses in its jurisdictions some of the holiest Christian sites in the world, including the church and convent of St Mary Magdalene on the Mount of Olives in Jerusalem, as well as the Alexander Court near the Church of the Holy Sepulchre, in which the only surviving gates of Jerusalem from the time of Jesus are carefully preserved. The Russian Church engages in active dialogue with the Russian state and public opinion, European institutions and the Vatican. After reunification, its voice in these discussions will become stronger.

On the other side of the ambo from where I was standing, a solemn President Vladimir Putin looked on. For him this was also a personal achievement of sorts: four years ago on a trip to the United States, he met Metropolitan Laurus and voiced his support for the reunification of the two parts of the Russian Church. He also helped to convince the metropolitan to visit Russia. This 2004 trip helped the head of Rocor to convince the majority of his clergy and laity to follow the course of unity.

"Today's revival of the Church's unity is a crucial precondition for restoring the unity of the entire Russian world, which has always seen Orthodoxy as its spiritual foundation," Mr Putin said after the signing ceremony. The reference to the "Russian world" was not accidental. Mr Putin spearheads the drive to reinvigorate ties between the Russian diaspora and Russia proper, as part of his relentless quest to regain influence on the global stage.

Professor Andrei Zubov, a prominent Moscow-based commentator on religious issues, said Rocor might bring into the life of the Russian Church as a whole a more collegial style, as well as a tendency to keep the state at arm's length. Andrew Phillips, a Rocor priest based in Felixstowe, Suffolk, said he was in favour of unity "because the devil is against it". Different parties pin different hopes on the momentous event of 17 May. Which ones are realised remains to be seen.