The Quran

109

فَلا تَكُ في مِريَةٍ مِمّا يَعبُدُ هٰؤُلاءِ ۚ ما يَعبُدونَ إِلّا كَما يَعبُدُ آباؤُهُم مِن قَبلُ ۚ وَإِنّا لَمُوَفّوهُم نَصيبَهُم غَيرَ مَنقوصٍ

Muhammad Asad

AND SO, [O Prophet,] be not in doubt about anything that those [misguided people] worship:¹ they but [thoughtlessly] worship as their forefathers worshipped aforetime; and, behold, We shall most certainly give them their full due [for whatever good or evil they have earned], without diminishing aught thereof.²

I.e., "do not think that their beliefs are based on reason": a reference, primarily, to the pagan Arabs who - like the wrongdoers spoken of in the preceding passages - rejected God's message on the plea that it conflicted with their ancestral beliefs; and, more generally, to all people who are accustomed to worship.(in the widest sense of this word) false values handed down from their ancestors and who, consequently, observe false standards of morality: an attitude which must unavoidably - as the last sentence of this verse shows - result in future suffering, be it in this world or in the hereafter, or in both.
Lit., "We shall repay them their portion in full, undiminished". For an explanation of this sentence, see note 27 on verses 15-16 of this surah.