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).
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