In August 1219 during the Fifth Crusade, Saint Francis of Assisi travelled to Damietta to meet sultan al-Kâmil of Egypt. The stories about this extraordinary event have been reinterpreted in different ways throughout the centuries. This article looks at the interpretation of the French islamologist Louis Massignon (1883-1962) and the way in which this story about ‘the saint and the sultan’ affects his vision of a dialogue between Islam and Christianity. Massignon played an important part in the changing attitude towards Islam in the Catholic Church before, during and after the Second Vatican Council. He based his interpretation of this encounter on Bonaventura’s Legenda Maior. Massignon is fascinated by Francis’ longing for martyrdom and his proposal to the sultan to undergo a trial by fire as a way of receiving God’s judgement. This divine judgement never takes place, but through his stigmata, Francis became a martyr to love after all. In Massignon’s mystical interpretation of the story, these stigmata are caused by Francis’ burning love for the Muslims. In 1934, Louis Massignon and Mary Kahil started the Badaliya-movement in Damietta. They vowed to sacrifice their lives for Muslims, ‘not so they would be converted, but so that the will of God may be performed in and through them’. Badaliya means ‘substitution’ and comes from the Arabic badal. In the tradition of the Sufi, the abdal are saints who are able to convert suffering by compassionately bearing it themselves. Massignon connects this Sufi tradition to the Catholic tradition of saints. These abdal reveal the sacred truth and enable a rapprochement between people from different religious traditions. From the perspective of contemporary, post-conciliar theology, the concept of mystical substitution poses all sorts of critical questions about the mysticism of sacrifice and suffering. However, the idea of self-sacrifice is a central theme in the life and works of Massignon. In the monastic inter-religious dialogue with Islam, we find traces of the concept of mystical substitution. Christian de Chergé’s spiritual testament, for example, shows that he was influenced by Massignon. Massignon’s importance for the future dialogue between Christians and Muslims should be found in the fact that his work is characterized by a great respect for Islam, while he firmly anchors the dialogue in the Catholic tradition by emphasizing mysticism, prayer, passion and the mystery of suffering.