The Cave
مَّا لَهُم بِهِۦ مِنْ عِلْمٍ وَلَا لِـَٔابَآئِهِمْ ۚ كَبُرَتْ كَلِمَةً تَخْرُجُ مِنْ أَفْوَٰهِهِمْ ۚ إِن يَقُولُونَ إِلَّا كَذِبًا
They have no knowledge of it, nor had their fathers. Grave is the word that comes out of their mouths; they speak not except a lie.
Nearby ayahs help place verse 18:5 in sequence and meaning.
مَّـٰكِثِينَ فِيهِ أَبَدًا
In which they will remain forever
وَيُنذِرَ ٱلَّذِينَ قَالُوا۟ ٱتَّخَذَ ٱللَّهُ وَلَدًا
And to warn those who say, "Allah has taken a son."
فَلَعَلَّكَ بَـٰخِعٌ نَّفْسَكَ عَلَىٰٓ ءَاثَـٰرِهِمْ إِن لَّمْ يُؤْمِنُوا۟ بِهَـٰذَا ٱلْحَدِيثِ أَسَفًا
Then perhaps you would kill yourself through grief over them, [O Muhammad], if they do not believe in this message, [and] out of sorrow.
إِنَّا جَعَلْنَا مَا عَلَى ٱلْأَرْضِ زِينَةً لَّهَا لِنَبْلُوَهُمْ أَيُّهُمْ أَحْسَنُ عَمَلًا
Indeed, We have made that which is on the earth adornment for it that We may test them [as to] which of them is best in deed.