People who do not accept the last prophet and/or the Qur'an are disbelievers, regardless of whether they believe in God or not!

Verse 68 of Chapter 29:

And who is more unjust than one who fabricates lies against Allah, or denies the truth* when it comes to him? Is there not a place in hell for disbelievers?  

(* The Holy Prophet and / or the Holy Qur’an).