Short answers, pulled from the story.
The Russian Orthodox Church traces its origins to 988, when Grand Prince Vladimir was baptized at Chersonesus in Crimea and began Christianizing his people. The Russian Orthodox Church declared autocephaly in 1448, electing its own metropolitan without the consent of the patriarch of Constantinople, marking the beginning of its formal independence.
The Russian Orthodox Church has more than 112 million adherents worldwide, making it the largest of all Eastern Orthodox churches. Among all Christian churches, it is second only to the Roman Catholic Church in number of followers.
The Soviet government confiscated church property, ridiculed religion, and propagated state atheism. Between 1917 and 1935, 130,000 Eastern Orthodox priests were arrested, and 95,000 of them were put to death. The number of functioning Orthodox churches fell from around 22,000 in 1959 to around 8,000 in 1965 during the Khrushchev-era persecutions.
The Russian Orthodox Church Outside Russia was established in the 1920s by Russian communities abroad who refused to recognize the authority of the Moscow Patriarchate under Soviet control. After decades of separation, the two churches reconciled on the 17th of May 2007, when an Act of Canonical Communion was signed in Moscow. ROCOR now functions as a self-governing entity within the Russian Orthodox Church.
On the 15th of October 2018, the Holy Synod of the Russian Orthodox Church severed full communion with the Ecumenical Patriarchate of Constantinople after Constantinople moved to end Moscow's jurisdiction over Ukraine and granted autocephaly to a new Orthodox Church of Ukraine, formalized on the 5th of January 2019. The Moscow Patriarchate had fiercely opposed Ukrainian autocephaly.
Patriarch Kirill of Moscow supported the invasion, calling it justified and blessing Russian soldiers fighting in Ukraine. In contrast, a group of 286 Russian Orthodox priests published an open letter on the 27th of February 2022 calling for an end to the war, and 200 priests from the Ukrainian Orthodox Church of the Moscow Patriarchate later called for Kirill to be tried for heresy. On the 20th of August 2024, the Ukrainian parliament banned the Russian Orthodox Church in Ukraine.