وَالْإِيمَانُ هُوَ الْإِقْرَارُ بِاللِّسَانِ وَالتَّصْدِيقُ بِالْجَنَانِ
Faith is profession by the tongue and conviction in the heart.
Inner assent is faith's essential core. Outward profession declares faith and carries worldly legal consequences. Someone who professes the testimony is publicly treated as a Muslim, while Allah alone fully knows the heart.
Faith is not reduced to habitual group membership or empty words alone.