What is the ruling on one who believes in the prophethood of Muhammad (peace be upon him) and that the Qur'an is the book of Allah, and acknowledges that Jesus is the servant of Allah and His messenger and not the son of Allah, yet still professes Christianity?

He who enters Islam and pronounces the two testimonies (Shahadatayn), but continues to attribute himself to his former religion (Jewish or Christian), his Islam is not accepted until he disavows his previous religion. Islam means disavowing all religions other than it, and merely uttering the two testimonies without disavowing the previous religion may not be sufficient, especially if he believes that Muhammad, peace be upon him, was sent only to the Arabs.

Allah Almighty says: "And whoever desires a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers." [Aal-Imran: 85]. And the Prophet, peace be upon him, said: "Whoever says: 'There is no god but Allah,' and disbelieves in what is worshipped besides Allah, his wealth and blood become inviolable, and his reckoning is with Allah." Narrated by Muslim.

Therefore, the new Muslim must declare his disavowal of his previous religion for his Islam to be considered sound and complete.