Why is the indefinite mubtadaʾ licensed in «وَلَعَبْدٌ مُؤْمِنٌ ...»?

Mubtadaʾ and Khabar — Definition and Foundational Rules

• Because it is an object
• It is qualified by «مُؤْمِنٌ»
• Because the predicate is genitive
• Because the first word is a verb