But if they do not respond to you - then know that (28:50)

﴿فَإِن لَّمْ يَسْتَجِيبُوا لَكَ فَاعْلَمْ أَنَّمَا يَتَّبِعُونَ أَهْوَاءَهُمْ ۚ وَمَنْ أَضَلُّ مِمَّنِ اتَّبَعَ هَوَاهُ بِغَيْرِ هُدًى مِّنَ اللَّهِ ۚ إِنَّ اللَّهَ لَا يَهْدِي الْقَوْمَ الظَّالِمِينَ﴾
[ القصص: 50]

28:50 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.

And if they do not take up your challenge to bring a book better guiding than the Torah and Qur’ān, know for certain that their rejection of the two is not based on any proof; it is only a result of following their own desires.
There is no one more deviant than the one who follows his desires without guidance from Allah, may He be glorified.
Allah does not grant a nation who wrongs itself through disbelief in Allah, opposing His Messengers and countering the truth that comes to them from Allah with falsehood, the ability to be guided.

But if they answer you not (i.e. do not believe in your doctrine of Islamic Monotheism, nor follow you), then know that they only follow their own lusts. And who is more astray than one who follows his own lusts, without guidance from Allah? Verily! Allah guides not the people who are Zalimun (wrong-doers, disobedient to Allah, and polytheists).

phonetic Transliteration

Fain lam yastajeeboo laka faiAAlam annama yattabiAAoona ahwaahum waman adallu mimmani ittabaAAa hawahu bighayri hudan mina Allahi inna Allaha la yahdee alqawma alththalimeena

Abdullah Yusuf Ali - Translation

But if they hearken not to thee, know that they only follow their own lusts: and who is more astray than one who follow his own lusts, devoid of guidance from Allah? for Allah guides not people given to wrong-doing.

Safi-ur-Rahman al-Mubarakpuri

But if they answer you not, then know that they only follow their own lusts. And who is more astray than one who follows his own lusts, without guidance from Allah Verily, Allah guides not the people who are wrongdoers.

Page 391 English transliteration