Islam has always taught that we must protect other religions, including monks and priests. Throughout history Christians have testified to the kind treatment of their priests and monks. For example, Isho-yahbh the Bishop, who was a Christian patriarch in the years 647 to 657 A.D., states,
“The Arabs [i.e. the Muslims], to whom God gave the dominion over the World, behave to us as you know. They are not hostile to Christianity, but praise our religion, honour the priests and saints, and help the Churches and Monasteries.”
The Prophet Muhammad (peace be upon him) taught his companions not to harm monks, and weak old men, even at times of war,
“Kill neither old man, nor little child, nor woman. Do not pull down a pillar saint from his place. Do not injure the monks, for they have set themselves apart to worship God.”
“Do not kill weak old men, small children or women.”
The kind treatment of Christians was also evident in the Prophet Muhammad’s (peace be upon him) treaty with the Christians of Najran,
“The lives of the people of Najran and its surrounding area, their religion, their land, their property, cattle, and those of them who are present or absent, their messengers and their places of worship are under the protection of God and guardianship of His prophet. Their present states shall neither be interfered with, nor their rights meddled with, nor their idols deformed. No bishop shall be removed from his office.”
These values were internalised by the Muslims through history. In 869 patriarch Theodosius of Jerusalem wrote about the goodwill and tolerance of the Muslims,
“The Saracens [i.e. the Muslims] show us great goodwill. They allow us to build our churches and to observe our own customs without hindrance.”
Even Bernard the Wise, a pilgrim monk, who visited Egypt and Palestine in the ninth century commented on the peace between Christians and Muslims,
“The Christians and the Pagans [i.e. the Muslims] have this kind of peace between them there that if I was going on a journey, and on the way the camel or donkey which bore my poor luggage were to die, and I was to abandon all my goods without any guardian, and go to the city for another pack animal, when I came back, I would find all my property uninjured: such is the peace there.”
The French priest was innocent. Islamic teachings strongly condemn the killing of one innocent person, which is viewed as equivalent to murdering the whole of mankind,
“The killing of a person for reasons other than legal retaliation or for stopping corruption in the land is as great a sin as murdering all of mankind.”
Let’s not forget that the Prophet Muhammad (peace be upon him) taught us to love for others what we love for ourselves,
“Love for humanity what you love for yourself.”
It is so sad that people like ISIS exist. They are deranged lunatics killing in the name of Islam. They are spiritually diseased and mentally unstable.
 The Book of Governors: The Historia Monastica of Thomas, Bishop of Marga A.D.840. (Edited From Syriac manuscripts In The British Museum And Other Libraries by E. A. Wallis Budge, Litt. D., F. S. A., – London: Kegan Paul, Trench, Trubner & Co., LTD. Paternoster House, Charing Cross Road. 1893), volume 2, p. 126.
 The Caliphs And Their Non-Muslim Subjects: A Critical Study Of The Covenant Of Umar (Humphrey Milford, Oxford University Press – London Bombay Calcutta Madras, 1930), by Arthur Stanley Tritton, p. 137; Tafsir Al Qurtubi, Classical Commentary Of The Holy Qur’an, (Dar al-Taqwa), volume 1, pp. 490-91.
 Narrated by Abu Dawud. Similar teachings are found in Muslims, At-Tirmidhi and Ibn Majah.
 Al-Baladhuri, Futuh al-Buldan, translated by Philip. K. Hitti, New Jeresy, 2002 (Reprint), pp. 100-1.
 Cited by Christopher J. Walker, Islam and the West, Gloucester, 2005, p. 17.
 The Qur’an, Chapter 5, Verse 32. Translation by Muhammad Sarwar.
 Narrated by Al-Bukhari, Tareekh Al-Kabeer.