What force does ba add in «wa-ma hum bi-muʾminin»?

The Rhetoric of Law: Nonoccurrence, Entailment, Hypothesis and Tense

• The whole clause after law is the response
• La-fashila is the negated condition while lam yajtahid is the result
• The condition is affirmative ijtahada and lam does not affect it
• Emphasis of the negative predicate