Salaam all
We know from brother Joseph's analysis, a disbeliever is a rejector of the truth and that each community has been given their own way, own scripture etc and that the Bible may not all be distorted.
However, in surah 5-15/16 is Allah not recommending the Quran instead of their old scripture???
Farther in 5-17, is Allah not calling all those who call jesus as God as "desbelievers"??
Other verses have clearly mentioned believers amongst the Christians, few in number, believing in one God, prostrating at night etc. There is a clear distinction between the two types of Christian believers and disbelievers regardless of the rejection of truth or not, maybe it's because God knows that they can't handle the truth. ALLAH THE ALMIGHTY knows best.
Salaam and peace