Bashar al-Assad's government expelled Dall'Oglio last month, three decades after he revived a monastery on a rocky outcrop overlooking the Syrian desert that became a centre for dialogue between the country's myriad ethnic and religious communities.
Nouri al-Jarrah, a London-based Syrian poet, called the expulsion a "shameful act". "He should be given Syrian nationality the day he returns," he said.
A big man with a loud voice and a calm manner, Dall'Oglio, 57, has reinvented himself as an unofficial diplomat on behalf of Assad's opponents abroad.
As a deeply-divided opposition movement tried to narrow their differences at a meeting in Cairo on Tuesday, the bearded Dall'Oglio was a key fixture, hurrying among the delegates and relaying messages from embattled activists back home.
"Assad's regime is so full of lies and spies that it no longer knows what is true or right," he told Reuters. "I am urging all diplomats I see to help the people and demanding that their countries force Assad to stop the violence and leave."
Admirers hope the priest can help achieve what Western powers have not - heal deep divisions between Assad's Muslim, Christian, Islamist and secularist opponents, who often seem united only by their hostility to Assad.
"I perceive faith as a bridge that we all must cross to be better people," he said. "The drive for power and personal glory is what makes people stray from religion and the extremists among them turn into tyrants like Bashar al-Assad."