In 1458, he was appointed as the metropolitan of the newly-established Kiev see by the titular Latin Patriarch Gregory Mammas of Constantinople on agreement of the Pope Pius II. Election Gregory was also supported by most of diocesan bishops of the original Old Russian Church among which were bishops of Przemysl, Chelm, Halych, Turow, Volodymyr, Lutsk, Polotsk, and Smolensk, while against were at least two metropolitan bishop in Moscow and Chernihiv. At least two more diocesan bishops of Great Novgorod and Tver chose to abstain in selection either side. In 1469 the Ecumenical Patriarch of Constantinople Dionysius I also gave his blessings approving the appointment of Gregory, thus Gregory in 1470 returned from the union.