assalamoalikum 
take this scenario… christian :C & Musalman :M
C: if u say that the bible was altered and has been altered many times then ur no worse than me! if u muslims believe that gods word can’t be altered then ur belief is also wrong!
M: i.e me gobsmacked
wat do i say to him he has a point that we believe gods word can’t be changed/altered, but isn’t it true that allah only promised to safe guard the quran for eternity.