BEIRUT, Lebanon — An Italian Jesuit priest who spent decades promoting religious dialogue in Syria and championed the uprising against President Bashar al-Assad embarked recently on a new mission: persuading an extremist Islamic group to release its prisoners and halt the battles that had spread violence across the country’s northeast.

That was a few weeks ago. He has not been heard from since, and unconfirmed reports that he has been killed have become increasingly common.

The disappearance of the priest, the Rev. Paolo Dall’Oglio, has worried Catholic leaders all the way up to Pope Francis, who has called for his release and offered prayers for his well-being. It has also struck many in the Syrian opposition as a dark symbol of where the uprising against Mr. Assad stands and how far some of its principal actors have deviated from the movement’s original aims.

“He kept saying to the people in the revolution that we can’t lose our goal of building a free, democratic Syria,” said Fawaz Tello, an opposition activist based in Germany who knows Father Dall’Oglio. But Mr. Tello said the priest had gone too far by seeking a cease-fire between Kurdish militias and the Islamic State in Iraq and Syria, which is linked to Al Qaeda.