«وَمَا هُمْ بِمُؤْمِنِينَ»
And they are not believers.
The negative nominal construction with ma and the additional ba in the predicate gives emphatic negation. It is relevant to the extended comparison because the discussion distinguishes state, persistence and emphasis in denying a state.
Do not collapse the bare negative proposition into its emphasized presentation. The pronoun identifies the subject; ma negates, and the predicate with ba strengthens the denial.