After Isidor had received funding from Vasili II, he went to Florence to attend an ecumenical council in 1439. He was made a cardinal-presbyter and a legate for the provinces of Lithuania, Livonia, all Russia and Galicia (Poland). During this holy meeting, Isidore was fervently defending the union between the Churches, but he was opposed by the only secular representative from Russia - ambassador Foma (Thomas) of Tver. Finally, the union agreement was signed and Isidor went to Russia.
The Russian princes denounced the union, but Isidor persisted. On his return from Italy, during his first divine service in the Cathedral of the Dormition in the Moscow Kremlin, Isidor was ordered to carry a Catholic cross in front of the ceremony, pray for the Pope during the mass, and read aloud the union agreement. Isidor passed a message to Vasili II from Pope Eugenius IV, containing a request to assist the Metropolitan in spreading Catholicism in Russia. Three days later Isidor was arrested and placed in the Chudov Monastery. He was condemned by the Russian clergy for refusing to repent and renounce the union.
In September of 1441, Isidor fled to Tver, then to Lithuania and Rome. In 1458, he was ordained as a nominal Patriarch of Constantinople.
MetropolitanIsidore and his retinue arrived in Moscow on March 19, 1441, and on that same day celebrated the Divine Liturgy in the Church of the Ascension in Moscow and promulgated the Union before Tsar Basil II and his court.
Metropolitan Andrew, due to the vicissitudes of the war, happened to have been a prisoner under house arrest in Russia at the time.
Shortly thereafter, on December 16, 1969 the then Metropolitan Alexei of Tallinn, now Patriarch Alexei II, acting as Director of Affairs of the MoscowPatriarchate, announced the Sacred Synod of the Russian Orthodox Church's decision to admit Catholics to receive communion in Russian Orthodox churches (this decision was subsequently rescinded several years later).