quotes from Tabari’s History, volume 10.
Abu Bakr’s letter to the apostates.
… So God guided with the truth whoever responded to Him, and the Apostle of God, with His permission, struck whoever turned his back to Him until he came to Islam, willingly or grudgingly.
… I have learned that some of you have turned back from your religion after you had acknowledged Islam and labored in it, out of negligence of God and ignorance of His command, and in compliance with the devil….
… I have sent you someone at the head of an army of the Muhajirun and the Ansar and those who follow (them) in good works. I ordered him not to fight anyone or to kill anyone until he has called him to the cause of God; so that those who respond to him and acknowledge (Him) and renounce (unbelief) and do good works, (my envoy) shall accept him and help him to (do right), but I have ordered him to fight those who deny (Him) for that reason. So he will not spare any one of them he can gain mastery over, (but may) burn them with fire, slaughter them by any means, and take women and children captive; nor shall he accept from anyone anything except Islam.
Another quote:
**Among them were many Christians who had accepted Islam, but when dissension had developed in Islam had said, “By God, our religion (din) from which we have departed is better and more correct than that which these people follow. Their religion does not stop them from shedding blood, terrifying the roads, and seizing properties.” **And they returned to their former religion. Al-Khirrit met them and said to them, "Woe unto you! Do you know the precept (hukm) of ‘Ali regarding any Christian who accepts Islam and then reverts to Christianity? By God he will not hear anything they say, he will not consider any excuse, he will not accept any repentance, and he will not summon them to it. His precept regarding them is immediate cutting off of the head when he gets hold of them. Those of the Banu Najiyah and other who were in that district came to him, and many men joined him.
I was in the army that ‘Ali b. Abi Talib sent against the Banu Najiyah. We came to them and found them split into three groups. Our commander said to one of these groups, “What are you?” and they replied, "We are a Christian people who do not consider any religion to be better than ours, and we hold fast to it. Our commander said to them, “Be off with you (i’tazilu)!” He said to another band, “What are you?” and they said, “We were Christians, but we accepted Islam and we hold fast to our Islam.” He said to them, “Be off with you!” Then he said to the third group, “What are you?” and they said, "We are a people who were Christians. We accepted Islam but we do not think, that any religion is better than our previous one." He said to them, “Accept Islam!” but they refused. He said to his men, "When I rub my head three times, attack them and kill the fighting men and make captive the dependants."
…But there was an **old man **among the, a Christian called al-Rumahis b. Mansur, who said, “By God the only error I have made since attaining reason was abandoning my religion, the religion of truth, for your, the religion of wickedness. No by God, I will not leave my religion and I will not accept yours so long as I live.” Ma’qil brought him forward and cut off his head."
Ma’qil wrote a letter to ‘Ali, the Caliph:
… For anyone who had apostatized, we offered return to Islam or else death. As for the Christians, we made them captive and led them off so that they might be a warning for those of the protected people who come after them not to refuse the jizyah and not to make bold against our religion and community, for the protected people are of little account and lowly in status.