Muslim or Non-Muslim, as per Quran attitude of rebellion (Istakbar/Sarkashi) is unforgivable in the sight of Allah.

إِنَّ الَّذِينَ كَذَّبُواْ بِآيَاتِنَا وَاسْتَكْبَرُواْ عَنْهَا لاَ تُفَتَّحُ لَهُمْ أَبْوَابُ السَّمَاء وَلاَ يَدْخُلُونَ الْجَنَّةَ حَتَّى يَلِجَ الْجَمَلُ فِي سَمِّ الْخِيَاطِ وَكَذَلِكَ نَجْزِي الْمُجْرِمِينَ لَهُم مِّن جَهَنَّمَ مِهَادٌ وَمِن فَوْقِهِمْ غَوَاشٍ وَكَذَلِكَ نَجْزِي الظَّالِمِينَ

Indeed those who denied Our revelations and evaded them in arrogance, the gates of heavens shall not be opened for them and neither shall they be able to enter Paradise except if a camel is able to pass through the eye of a needle. [This is their punishment] and in this manner do We punish the criminals. Hell shall be their bedding and [the flames of] Hell shall be their covering and in this way do We punish the evil-doers. (7:40-41)

Therefore, a true muslim can never commit istakbar. If he commits sins, he will revert back to God and asks forgiveness which will ultimately result in his forgivenss, but if there is even iota of arrogance in one’s heart, then there is no forgiveness for such a person. So as long as a Muslim stayed away from a rebellious attitude, then chances are that He/She will eventually go to Jannah after Tazkiya, if required (as a result of his true eman).

Please refer to the following responses of Ghamidi Sahab:

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