Short answers, pulled from the story.
Roman Emperor Constantine convened the First Council of Nicaea in 325 to address theological disputes threatening the early Christian community. Over three hundred bishops gathered in Nicaea to condemn Arius and establish the Nicene Creed which remains a foundational statement for Eastern Orthodox Christians today.
The Seventh Ecumenical Council met in Constantinople on the 787 to support the veneration of icons while forbidding their worship. This event is often referred to as The Triumph of Orthodoxy and defined the proper use of religious images within the church.
It is estimated that 50,000 clergy had been executed between the revolution and the end of the Khrushchev era. During this period members of the church hierarchy were jailed or forced out of their places and replaced by docile clergy with ties to the KGB.
On the 15th of October 2018 the Holy Synod of the Russian Orthodox Church severed all ties with the Ecumenical Patriarchate of Constantinople. This action barred all members of the Russian Church from receiving communion or sacraments from any churches with ties to the Ecumenical Patriarchate.
As of 2012 the Eastern Orthodox Church had approximately 300 million adherents making it the third largest religious community in the world after Catholics and Sunni Muslims. Over two-thirds of all Eastern Orthodox members are concentrated in Southeast Europe Eastern Europe and Russia with significant minorities in Central Asia and the Levant.