Must a Christian who believes that Jesus and Muhammad are both Messengers of God follow Islamic Sharia, in accordance with His saying, Exalted is He: "And whoever desires as religion other than Islam, it will never be accepted from him"?

Whoever remained among the People of the Book upon their religion after the advent of Muhammad (peace be upon him) is not upon a sound religion. Their mere knowledge that Muhammad is the Messenger of Allah does not constitute Islam unless it is accompanied by obedience to him and submission to what he brought. The People of the Book whom the Quran declared disbelievers and who recognized the Messenger, that recognition did not protect their lives, for Allah Almighty said: "Those to whom We gave the Scripture know him as they know their own sons. But indeed, a party of them conceal the truth while they know it." No one enters Islam except by uttering the two testimonies of faith (Shahadatayn) and fulfilling their conditions, which include submission to what has come from Allah and His Messenger. For religion is obedience and submission, and whoever does nothing for Allah has no religion.