His statement: “He is not to be allowed to remain, and nothing is accepted from him except Islam or his former religion.” That is, if a Christian becomes Jewish, we say to him: We do not accept this from you. Either embrace Islam or return to the religion from which you departed.
If a Christian becomes a Magian, then this applies all the more so; likewise, if a Magian becomes Christian. Thus, anyone who leaves the religion he was following is told: Either return to it or embrace Islam. This is because he cannot move to a religion that is better in terms of its acceptance by Allah. All religions other than Islam are unacceptable to Allah, so there is no benefit in his changing religions.
Likewise, if a Jew becomes Christian, we do not accept it from him. We say: Either return to Judaism or embrace Islam.
Some scholars have said that if a Christian becomes Jewish, nothing is accepted from him except Islam, because his departure from Christianity for Judaism is an acknowledgment on his part that Christianity is false, while he has moved to another false religion. Thus, the religion you previously followed is false, and the one to which you have moved is likewise false. We cannot allow you to remain upon falsehood; rather, we say: Embrace Islam, or we will kill you. There is no doubt that this view has strong grounds.
If he refuses, saying, “I will neither return nor embrace Islam,” then he is to be killed, because he is now upon an invalid religion. However, Imam Ahmad, may Allah have mercy on him, was asked about this and withheld judgment, or said: He is not to be killed. Yet what appears to be indicated by the evidence is that he is to be killed, since he is now following a religion upon which he cannot be allowed to remain. But Imam Ahmad, may Allah have mercy on him, held that he possesses the protected status and covenant of a non-Muslim subject, and that this does not invalidate his protected status or covenant; therefore, he may not be killed. This is the basis of Imam Ahmad’s view that we do not kill him.