You can only warn one who follows the message and fears the Most Merciful unseen. So give him good tidings of forgiveness and noble reward.
The Saheeh International Quran Translation
إِنَّمَا تُنذِرُ مَنِ ٱتَّبَعَ ٱلذِّكْرَ وَخَشِىَ ٱلرَّحْمَٰنَ بِٱلْغَيْبِ فَبَشِّرْهُ بِمَغْفِرَةٍ وَأَجْرٍ كَرِيمٍYou can only warn one who follows the message and fears the Most Merciful unseen. So give him good tidings of forgiveness and noble reward.