فَمَنْ صَدَّقَ بِقَلْبِهِ وَلَمْ يُقِرَّ بِلِسَانِهِ، فَهُوَ مُؤْمِنٌ عِنْدَ اللَّهِ
Whoever truly assents inwardly without professing verbally is a believer before Allah.
This formulation turns on the existence of genuine inward assent, not mere familiarity or an unsupported claim. An absent declaration can complicate worldly recognition without demonstrating that no assent exists in the person's heart.
However, deliberate refusal to profess without excuse has its own legal and theological discussion. No one may confidently determine an individual's inner state through guesswork alone.