Does not His Almighty's saying: "And Muhammad is but a messenger; messengers have passed away before him. If then he dies or is killed, will you turn back on your heels?" indicate that Allah did not know whether the Prophet would die or be killed?

Allah is the Knower of the unseen and knows the secret and what is more hidden. It is mentioned in the Du'a of Istikhara: "For You are able and I am not; You know and I do not, and You are the Knower of the unseen." And the verse: ﴿If he dies or is killed, will you turn back on your heels?﴾ does not mean that Allah does not know that he will not be killed. Rather, the mention of ﴿killed﴾ is according to the supposition of those addressed, because the messengers before Prophet Muhammad (peace be upon him) died and were killed. The purpose is to urge adherence to the religion and to jihad, and not to generate doubts about clear meanings in the Qur'an.