(Metropolitan Hilarion, head of the Russian Orthodox Church's foreign relations department, attends an interview with Reuters in Moscow May 16, 2014. Religious tensions are deepening dividing lines in Ukraine's crisis, with rival churches taking political sides and Kiev slighting the Russian Orthodox Church, Metropolitan Hilarion said on Friday. REUTERS/Maxim Shemetov)

(Metropolitan Hilarion during an interview with Reuters in Moscow May 16, 2014. REUTERS/Maxim Shemetov)

When Pope Francis meets the spiritual head of the world’s Orthodox Christians next week, the speeches and symbolism will focus on how these ancient western and eastern wings of Christianity want to come closer together.

After almost a millennium apart, however, the key to the elusive unity they seek does not lie in Jerusalem, where the Catholic pope and Ecumenical Patriarch Bartholomew will embrace on May 25. If anywhere, that key lies in Moscow.

The Russian Orthodox Church (ROC), by far the largest church in the Orthodox world and increasingly influential at home and abroad, has long been wary of the closer ties Francis and Bartholomew want to work towards.

Its opposition has only stiffened in recent months amid the crisis in Ukraine, where the political standoff between Russia and Europe has deepened tensions between the Moscow Patriarchate and three competing churches, one of them linked to the Vatican which Moscow accuses of trying to woo away its worshippers.