You are reading a tafsir for the group of verses 40:28 to 40:31
وقال رجل مومن من ال فرعون يكتم ايمانه اتقتلون رجلا ان يقول ربي الله وقد جاءكم بالبينات من ربكم وان يك كاذبا فعليه كذبه وان يك صادقا يصبكم بعض الذي يعدكم ان الله لا يهدي من هو مسرف كذاب ٢٨ يا قوم لكم الملك اليوم ظاهرين في الارض فمن ينصرنا من باس الله ان جاءنا قال فرعون ما اريكم الا ما ارى وما اهديكم الا سبيل الرشاد ٢٩ وقال الذي امن يا قوم اني اخاف عليكم مثل يوم الاحزاب ٣٠ مثل داب قوم نوح وعاد وثمود والذين من بعدهم وما الله يريد ظلما للعباد ٣١
وَقَالَ رَجُلٌۭ مُّؤْمِنٌۭ مِّنْ ءَالِ فِرْعَوْنَ يَكْتُمُ إِيمَـٰنَهُۥٓ أَتَقْتُلُونَ رَجُلًا أَن يَقُولَ رَبِّىَ ٱللَّهُ وَقَدْ جَآءَكُم بِٱلْبَيِّنَـٰتِ مِن رَّبِّكُمْ ۖ وَإِن يَكُ كَـٰذِبًۭا فَعَلَيْهِ كَذِبُهُۥ ۖ وَإِن يَكُ صَادِقًۭا يُصِبْكُم بَعْضُ ٱلَّذِى يَعِدُكُمْ ۖ إِنَّ ٱللَّهَ لَا يَهْدِى مَنْ هُوَ مُسْرِفٌۭ كَذَّابٌۭ ٢٨ يَـٰقَوْمِ لَكُمُ ٱلْمُلْكُ ٱلْيَوْمَ ظَـٰهِرِينَ فِى ٱلْأَرْضِ فَمَن يَنصُرُنَا مِنۢ بَأْسِ ٱللَّهِ إِن جَآءَنَا ۚ قَالَ فِرْعَوْنُ مَآ أُرِيكُمْ إِلَّا مَآ أَرَىٰ وَمَآ أَهْدِيكُمْ إِلَّا سَبِيلَ ٱلرَّشَادِ ٢٩ وَقَالَ ٱلَّذِىٓ ءَامَنَ يَـٰقَوْمِ إِنِّىٓ أَخَافُ عَلَيْكُم مِّثْلَ يَوْمِ ٱلْأَحْزَابِ ٣٠ مِثْلَ دَأْبِ قَوْمِ نُوحٍۢ وَعَادٍۢ وَثَمُودَ وَٱلَّذِينَ مِنۢ بَعْدِهِمْ ۚ وَمَا ٱللَّهُ يُرِيدُ ظُلْمًۭا لِّلْعِبَادِ ٣١
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3

In verse 28, it was said: يَكْتُمُ إِيمَانَهُ (who had kept his faith secret). This tells us that a person who does not declare his 'iman (faith) before people, yet remains staunch in his faith by heart, then, this person is a believer. But, it stands proved from clear textual authority (of the Qur'an and Hadith) that, for 'iman to be acceptable, the simple certitude of the heart is not enough, instead, it is subject to the condition of a verbal confession and declaration. Unless the person concerned declares it verbally, he or she will not be regarded as a believer. However, making this verbal declaration before people publicly is not necessary. The only reason why it is needed is that unless people come to know about the person's 'iman, they would remain unable to interact with him or her in the same way as they do with Muslims. (Qurtubi)

Earlier in the verse, by saying: مُّؤْمِنٌ مِّنْ آلِ فِرْ‌عَوْنَ (a believing man from the House of the Pharaoh), it is virtually demonstrated that the believer, in his ensuing dialogue with Pharaoh and his people, invited them toward truth and faith as well as restrained them from killing Sayyidna Musa (علیہ السلام) .