Tafseer of The Cave · Al-Kahf · 18:104
[They are] those whose effort is lost in worldly life, while they think that they are doing well in work."
Important: The Arabic source text is always authoritative. This translation is a study aid and has not been verified by scholars — do not use it as a basis for religious proof or for deriving rulings (ahkam). When in doubt, always consult the Arabic text and a qualified scholar.
The exposition of the meaning of the words: الَّذِينَ ضَلَّ سَعْيُهُمْ فِي الْحَيَاةِ الدُّنْيَا (Those whose striving was lost in the worldly life) — that is to say: they are those whose works which they performed in their worldly life were not upon rightly-guided and just grounds, but upon errant and wrong grounds. This is because they acted contrary to what Allah had commanded them, but out of disbelief (kufr) toward Him. وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا — that is to say: while they suppose that by that action they are obeying Allah and are diligent in what He has recommended to His servants. This is one of the clearest proofs of the falseness of the opinion of the one who claims that no one becomes a disbeliever (kāfir) toward Allah unless he knowingly chooses disbelief after knowledge of His oneness (waḥdāniyya) — for Allah, the Exalted, has informed concerning those whose characteristics He has described in this verse that their striving which they did in the worldly life was lost, while they supposed that they were acting well. And He informed concerning them that they are those who denied the signs (āyāt) of their Lord. Were it as those claimed who said that no one becomes a disbeliever toward Allah unless he knows it, then these people with their works — concerning which Allah has informed that they supposed therein that they were acting well — would have been rewarded and recompensed. But it is the contrary of what they say. Allah, whose praise is exalted, has informed concerning them that they are disbelievers toward Allah and that their works are rendered void (ḥabiṭa).
By His words أَنَّهُمْ يُحْسِنُونَ صُنْعًا He means: an action (ʿamalan). The words "ṣunʿ," "ṣinʿa," and "ṣanīʿ" have the same meaning. One says: "a well-tended horse" (faras ṣanīʿ) in the meaning: "a well-cared-for horse."