The Russian Orthodox Church has already declared them martyrs for the faith, releasing the short video above.
Two Russian mercenaries reportedly captured by ISIS jihadists are feared to have been executed because they refused to read out a statement saying they rejected their Christian faith and had become Muslim.
Roman Zabolotny, 39, and Grigory Tsurkanu, 38, held in Syria, also refused to say on camera that they had joined the terrorist group.
The men are believed to have been fighting for a private Russian mercenary force comprising ex-servicemen which saw action in Crimea and eastern Ukraine before Syria.
They were held by jihadists around the eastern city of Deir Ezzor, the last ISIS stronghold in the country.
They appeared in a video released earlier this week by ISIS news agency Amak which claimed inaccurately that the men were serving Russian soldiers.
Senior Russian MP Viktor Vodolatsky said: ‘It is very sad but 99 per cent Roman Zabolotny is not alive, nor is the second prisoner.
‘Before filming that video they were given a statement which they had to read.
‘In this text they would reject their Orthodox religion, reject their motherland, become Muslim and join ISIS.
‘They stayed loyal to the Orthodox faith and their Motherland until the very end, and this is what they were killed by those gangsters for.’
There has been no official confirmation from the Russian authorities about their fate, but Vladimir Putin’s government is coy speaking about such mercenary forces in Syria and other hot spots, even though experts say they work closely with the armed forces.