The Quran

مَثَلُ الَّذينَ كَفَروا بِرَبِّهِم ۖ أَعمالُهُم كَرَمادٍ اشتَدَّت بِهِ الرّيحُ في يَومٍ عاصِفٍ ۖ لا يَقدِرونَ مِمّا كَسَبوا عَلىٰ شَيءٍ ۚ ذٰلِكَ هُوَ الضَّلالُ البَعيدُ

Muhammad Asad

[This, then, is] the parable of those who are bent on denying their Sustainer: all their works¹ are as ashes which the wind blows about fiercely on a stormy day: [in the life to come,] they cannot achieve any benefit whatever from all [the good] that they may have wrought: for this [denial of God] is indeed the farthest one can go astray.²

I.e., even the good ones (Razi).
Lit., "this, this is the straying far-away". The definite article in the expression ad-dalal al-ba'id, preceded by the pronouns dhalika huwa, is meant to stress the extreme degree of this "straying far-away" or "going astray": a construction that can be rendered in English only by a paraphrase, as above. It is to be noted that this phrase occurs in the Qur'an only twice-namely, in the above passage and in 22:12 - and refers in both cases to a denial, conscious or implied, of God's oneness and uniqueness.