Which popes belonged to religious orders?

Answer 1

38 popes have been members of religious orders. These have included:

Benedictines (18):

Gregory I, Boniface IV, Adeodatus II, Leo IV, John IX, Leo VII, Sergius IV, Stephen IX, Gregory VII, Victor III, Urban II, Paschal II, Gelasius II, Celestine V, Clement VI, Urban V, Pius VII, Gregory XVI

Augustinians (6):

Eugene IV, Honorius II, Innocent II, Lucius II, Gregory VIII, Adrian IV

Dominicans (5):

Innocent V, Benedict XI, Nicholas V, Pius V, Benedict XIII

Franciscans (5):

Nicholas IV, Sixtus IV, Julius II Sixtus V, Clement XIV

Cistercians (2):

Eugene III, Benedict XII

Theatines (1):

Paul IV

Jesuits (1):

Francis

12 popes have been members of third orders, also called "tertiaries" or "Third Order Secular," and all of them of the Secular Franciscans:

Gregory IX, Gregory X, Martin V, Innocent XII, Clement XII, Pius IX, Leo XIII, Pius X, Benedict XV, Pius XI, Pius XII, John XXIII