When a Muslim cleric criticized the Nusra Front last year for taking over his Syrian city and raising its menacing black flags, a representative of the jihadist group took to Facebook to send him an ominous message.

“Oh secularist, oh infidel,” the note read. “Sit quietly or your time will come.”

Yet when wider protests over Nusra’s draconian practices and rigid religious views soon followed in cleric Murhaf Shaarawi’s home city of Maraat Numan and elsewhere in Idlib province, the group took note. It curbed its threats to clerics and its attempts to spread its brand of Islam, said Mr. Shaarawi and other current and former residents of the province. […]