Acknowledgement is not sufficient as long as the person does not embrace Islam
15-2-2012 | IslamWeb
Question:
There is a man who adopts the sound Christian religion, not the distorted one. He believes in Allaah Alone, Muhammad and all other messengers, the Angels, the Last Day, the Quran and all other Divine Books, and fate. In addition, He uttered the two testimonies of faith but he did not embrace Islam. However, he believes in the Oneness of Allaah. Will he enter Paradise after reckoning or Hell?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If we suppose that there is a man who adopts the sound Christian religion which is not distorted, believes in the Oneness of Allaah and has all such characteristics that was mentioned by the questioner, then this acknowledgement is not sufficient at all until he adopts the Islamic religion, believes in Muhammad, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, and abides inwardly and outwardly by the Islamic Sharee’ah (Islamic law). Allaah The Almighty Says (what means): {And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers.} [Quran 3:85]
It is narrated on the authority of Abu Hurayrah, 
that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “By the One in Whose Hand is the life of Muhammad (i.e. Allaah), he who amongst the community of Jews or Christians hears about me, but does not affirm his belief in that with which I have been sent and dies in this state (of disbelief), he shall be but one of the dwellers of Hell.” [Muslim]
Furthermore, if this person was a true follower as you have mentioned in your question, he would have surely believed in Muhammad, sallallaahu ‘alayhi wa sallam, and embraced Islam. This is because ‘Eesa (Jesus) 
brought forth the glad tidings of the advent of Prophet Muhammad, sallallaahu ‘alayhi wa sallam, and the obligation of believing in him, and that whoever does not do accordingly has in fact not followed ‘Eesa .
Allaah Knows best.