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فَإِن لَّمْ يَسْتَجِيبُوا۟ لَكَ فَٱعْلَمْ أَنَّمَا يَتَّبِعُونَ أَهْوَآءَهُمْ ۚ وَمَنْ أَضَلُّ مِمَّنِ ٱتَّبَعَ هَوَىٰهُ بِغَيْرِ هُدًى مِّنَ ٱللَّهِ ۚ إِنَّ ٱللَّهَ لَا يَهْدِى ٱلْقَوْمَ ٱلظَّـٰلِمِينَ
But if they do not respond to you - then know that they only follow their [own] desires. And who is more astray than one who follows his desire without guidance from Allah? Indeed, Allah does not guide the wrongdoing people.
Nearby ayahs help place verse 28:50 in sequence and meaning.
فَلَمَّا جَآءَهُمُ ٱلْحَقُّ مِنْ عِندِنَا قَالُوا۟ لَوْلَآ أُوتِىَ مِثْلَ مَآ أُوتِىَ مُوسَىٰٓ ۚ أَوَلَمْ يَكْفُرُوا۟ بِمَآ أُوتِىَ مُوسَىٰ مِن قَبْلُ ۖ قَالُوا۟ سِحْرَانِ تَظَـٰهَرَا وَقَالُوٓا۟ إِنَّا بِكُلٍّ كَـٰفِرُونَ
But when the truth came to them from Us, they said, "Why was he not given like that which was given to Moses?" Did they not disbelieve in that which was given to Moses before? They said, "[They are but] two works of magic supporting each other, and indeed we are, in both, disbelievers."
قُلْ فَأْتُوا۟ بِكِتَـٰبٍ مِّنْ عِندِ ٱللَّهِ هُوَ أَهْدَىٰ مِنْهُمَآ أَتَّبِعْهُ إِن كُنتُمْ صَـٰدِقِينَ
Say, "Then bring a scripture from Allah which is more guiding than either of them that I may follow it, if you should be truthful."
۞ وَلَقَدْ وَصَّلْنَا لَهُمُ ٱلْقَوْلَ لَعَلَّهُمْ يَتَذَكَّرُونَ
And We have [repeatedly] conveyed to them the Qur'an that they might be reminded.
ٱلَّذِينَ ءَاتَيْنَـٰهُمُ ٱلْكِتَـٰبَ مِن قَبْلِهِۦ هُم بِهِۦ يُؤْمِنُونَ
Those to whom We gave the Scripture before it - they are believers in it.