Jacques Mourad of the Syriac Catholic Church, who was abducted by unknown gunmen in May, said the "hardest thing" was hearing his captors pledge to kill him if he did not covert to Islam.
The 48-year-old, who was kidnapped along with another Syrian from the Mar Moussa monastery north of Damascus by masked gunmen, described to journalists in Rome the moment of his abduction.
A man dressed in black, "similar to the ones you see in the IS videos who decapitate their prisoners", turned up at the door. "I thought, 'the end has arrived'," he said.
They were locked up in a small bathroom, blindfolded and with their hands tied.
But the man told Mourad "you are under my protection" -- he thinks thanks to the reputation of the Mar Moussa monastery, where both Christians and Muslims are welcomed and helped.
The experience was "a very intense time on a spiritual level," he said.
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In early August, Mourad was transferred from his prison to join another 250 or so Christians captured by IS when the group overran his parish of Al-Qaryatain.
He said IS leader Abu Bakr al-Baghdadi said it could go several ways: the jihadists could "kill the men and take the women and children; enslave them all; ask for ransom; or give 'the gift of life'" to the captured Christians, letting them return to their violence-hit homes.
They were freed after signing a document on September 1, which said they agreed to live under IS authority.
Al-Qaryatain had been reduced to a wasteground, with no water, electricity or food. Mourad celebrated mass in a basement "almost daily, to avoid the bombing".
On October 10, he and his flock decided to leave the city because "life under IS is impossible for Christians", but eight members of the group died as the community -- which included elderly people, children and disabled people -- fled for freedom.
Mourad will meet Pope Francis next week, having completed a pilgrimage to the holy site of Lourdes in France to thank the Virgin Mary for having survived. After that, he plans to return to Syria.
Not, however, before reminding Islamic extremists that "their religion is one of mercy" and calling on the West to fulfil its "responsibilities to Syrians fleeing the bombings and massacres, who die at sea in their attempt to escape".