Surah Kahf

What is the central theme of this Surah? Why is it recommended to ward off the ultimate trial of ad-Dajjal (the anti-Christ)?

Salvific exclusivity before and after Prophet Muhammad (saws). Quran 2:62

What is the final destination of people who believe in Allah, day of Judgment and performed good deeds, before Mohammad (saws) prophecy and after.

إِنَّ الَّذِينَ آمَنُوا وَالَّذِينَ هَادُوا وَالنَّصَارَىٰ وَالصَّابِئِينَ مَنْ آمَنَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ وَعَمِلَ صَالِحًا فَلَهُمْ أَجْرُهُمْ عِندَ رَبِّهِمْ وَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ