This is the 12771st most frequent Polish word.
niewierzący
"Non-believer."
The word 'niewierzący' describes someone's lack of religious beliefs here.
Adam jest niewierzący i nie uczestniczy w rytuałach religijnych.
Adam is non-religious and does not participate in religious rituals.
In this context, 'niewierzący' is a noun collectively referring to people who lack belief in religion.
Niewierzący mogą mieć różne spojrzenie na życie niż osoby religijne.
Non-believers can have a different outlook on life than religious people.
Again, 'niewierzący' is used adjectively to describe someone's individual characteristic of not adhering to religious belief.