God's religion is one. Whether it is called Christianity or Judaism or Islam, it all calls for worshiping God alone, surrendering to God, and believing in the day of judgment. When Judaism started, it was the only accepted religion of its time. When Jesus was sent, Christianity became the religion of God. Anyone who rejects Jesus is considered a disbeliever. When Islam came, it became the only accepted religion. Anyone who rejects Islam is considered a disbeliever in God. Some scholars add to that those people who believe in God alone and the day of Judgment, but never heard of Islam or never got the message of Islam in a clear way.
As for those Christians who believe in Jesus as God or the son of God, God says what can be translated to mean:5:116 And behold! Allah will say: "O Jesus the son of Mary! Didst thou say unto men, worship me and my mother as gods in derogation of Allah.?" He will say: "Glory to Thee! never could I say what I had no right (to say). Had I said such a thing, thou wouldst indeed have known it. Thou knowest what is in my heart, Thou I know not what is in Thine. For Thou knowest in full all that is hidden.
5:117 "Never said I to them aught except what Thou didst command me to say, to wit, 'worship Allah, my Lord and your Lord'; and I was a witness over them whilst I dwelt amongst them; when Thou didst take me up Thou wast the Watcher over them, and Thou art a witness to all things.
5:118 "If Thou dost punish them, they are Thy servant: If Thou dost forgive them, Thou art the Exalted in power, the Wise."
5:119 Allah will say: "This is a day on which the truthful will profit from their truth: theirs are gardens, with rivers flowing beneath,- their eternal Home: Allah well-pleased with them, and they with Allah. That is the great salvation, (the fulfilment of all desires).