Surah an-Nisa' 4:150-151

   Lo! those who disbelieve in Allah and His messengers, 
   and seek to make distinction between Allah and His messengers, 
   and say: We believe in some and disbelieve in others, 
   and seek to choose a way in between;
   Such are disbelievers in truth; 
   and for disbelievers We prepare a shameful doom.

This passage makes clear that those who reject Muhammad are declared kuffar. Despite the other verses that speak about the people of the book more nicely, this passage puts it rather unambiguously blunt.

This is obvious from the context since the sermon continues with denouncing the people of the book for various errors and then states:

162:  But the firm in knowledge among them and the believers 
      believe in what has been revealed to you [i.e. Muhammad]
      and what was revealed before you, ...
163:  Surely We have revealed to you [i.e. Muhammad] 
      as We revealed to Nuh, and the prophets after him, 
      and We revealed to Ibrahim and Ismail and Ishaq and 
      Yaqoub and the tribes, and Isa and Ayub and Yunus and 
      Haroun and Sulaiman and We gave to Dawood the Psalms.

Conclusion: By rejecting Muhammad, Christians and Jews become disbelievers, including "a shameful doom" as future (i.e. Hell).

Further discussion of these verses on the newsgroups: [1], [2], [3]

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