Ibn Ishaq narrates from Muhammad bin Ja'far bin al-Zubayr that he said to me: Do you know why Abu Bakr (may Allah be pleased with him) made a distinction, ordering the killing of the church attendant who tends the church (the deacon), while forbidding the killing of the monk? I said: In my view, it is only because these (monks) have confined themselves. He said: Yes; but the deacons (shamamisah) engage in fighting and fight themselves, whereas the way of the monks is not to fight, and Allah Almighty says: "And fight in the path of Allah those who fight you." [Surah al-Baqarah: 190]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18151