Thousands of Orthodox Christian believers marched and chanted through the streets of Moscow, Kyiv, and other cities, marking the 1,030th anniversary of the adoption of Christianity in the region.
The July 28 events commemorated the date in 988 of the baptism of Prince Vladimir, who was the leader of a federation of Slavic tribes known as Kievan Rus.
Centuries later, the federation evolved into the Russian Empire.
Underscoring how deeply politics have become intertwined with religion, Russian President Vladimir Putin spoke to thousands of clergy and believers in Moscow, at a statue dedicated to Prince Vladimir.
Adopting Christianity was "the starting point for the formation and development of Russian statehood, the true spiritual birth of our ancestors, the determination of their identity. Identity, the flowering of national culture and education," Putin said.
The head of the Russian Orthodox Church, Patriarch Kirill, referenced the fact that Putin shares a first name with Prince Vladimir.
"I think there is no such thing as a coincidence, especially when we are talking about people whose actions truly change the world," Kirill told Putin.
The location where Prince Vladimir was reportedly baptized is located on the Crimean Peninsula, the Black Sea region in what is now Ukraine. Russia seized Crimea in 2014, and Putin has in the past referenced the baptismal site as part justification for the unrecognized annexation.
In Kyiv, meanwhile, thousands of people marched through the streets carrying religious icons and Ukrainian flags.
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Russian Orthodox Church In Ukraine Marks Christianization Of Kievan Rus
Ukrainian President Petro Poroshenko condemned the Russian Orthodox Church, calling it a threat to national security.
The Orthodox Church in Ukraine is split between the largest branch whose clerics pledge loyalty to the head of the Russian church, and one that is overseen by Kyiv-based patriarch.