HOMS, Syria—A handful of Jesuit priests and nuns huddled with local residents Sunday for Easter services here inside a modest black stone church, with thunderous explosions and the crackle of gunfire sounding in the background.

Many were teary-eyed as they remembered Father Frans van der Lugt, a leader of their community who was shot and killed in his monastery in the besieged, rebel-held old quarter of Homs two weeks ago.

The...