Ilarion said that Constantinople's decision to back what the Russian Orthodox Church considers a schismatic church is "illegal and canonically worthless", and effectively drives it into a schism.
On 15 October, the Istanbul-based leader of the Eastern Orthodox Church, Ecumenical Patriarch I Bartholomew of Constantinople, took the monumental decision to recognise the full independence of the Ukrainian Orthodox Church, after a three-day Synod.
When it gained independence in 1991 following the fall of the Soviet Union, Ukraine strove to establish religious independence and an Orthodox Church of the Kiev Patriarchate was created.
The Russian Orthodox Church's Synod will take an unambiguous stance on the decisions taken in Istanbul, Volkov said.
Ukrainian President Petro Poroshenko said the Church's independence went hand-in-hand with Ukrainian independence.
Constantinople's decision seems to be serving the interests of the Ukrainian leadership rather than the Orthodox Christians living there.
The move comes following the announcement of the Constantinople Patriarchate on Thursday that it would proceed to the granting independence to the Orthodox Church in Ukraine, which is now split into three major churches - the Ukrainian Orthodox Church (UOC) of Moscow Patriarchate, the non-canonical Ukrainian Orthodox Church of Kiev Patriarchate (UOC-KP), which was created after the collapse of the Soviet Union, and the so-called Ukrainian Autocephalous Orthodox Church.
The initiative has been strongly criticized by the Russian Orthodox Church, which moved on to the actual "breaking of diplomatic relations" with the Patriarchate of Constantinople, suspending service with its hierarchs.
Ukraine accuses the Russian Orthodox Church of wielding a pernicious influence on its soil, allowing itself to be used as a tool of the Kremlin to justify Russian expansionism and support of separatist rebels in eastern Ukraine. Meanwhile, it did not hesitate to seize Moscow Patriarchate's churches by force.
A spokesman for Russian Patriarch Kirill said it "crossed a red line" and is "catastrophic" for the entire Orthodox world.
"A schism remains a schism".
In addition, Synod announced earlier that it lifted the anathema from the heads of the two non-canonical churches that had been excommunicated by the Russian Orthodox Church for their schism.
According to TASS, 40 churches have been forcefully seized by the Kiev Patriarchate between 2014 and 2016.
The Belarusian Orthodox Church, which is by far the biggest religious denomination in the nation of almost 10 million, is under the direct jurisdiction of the Russian Orthodox Church.
Constantinople had previously rejected the Ukrainian Church's independence moves, but made a decision to endorse them last week.
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