ROME—Pope Francis has called a global meeting of Catholic bishops to discuss how to prevent sex abuse, an unprecedented move and a sign that the pontiff is trying to retake the initiative on an issue that has thrown his pontificate into crisis.

No pope has ever called such a large-scale meeting of church leaders specifically to address the sex-abuse crisis. The meeting, set for February, is a high-stakes gamble that instantly raises expectations of overhauls in the church and sets up the pope to take the blame if expectations...