Reminder of Blessings Here
Allah reminds His creation of the blessings He bestows upon them in this world and the Hereafter. In the heavens He has subjugated for them the stars which give them light during the night and during the day, and He has created clouds, rain, snow and hail, and made the heavens a canopy which covers and protects them. On earth He has created for them stability and rivers, trees, crops and fruits; He has completed and perfected His graces upon them, apparent and hidden, by sending Messengers, revealing Books and removing doubts and excuses. Yet despite all this, not all the people believe, and indeed there are those who dispute concerning Allah, that is, His Tawhid, and His sending the Messengers. Their dispute is without knowledge and they have no sound evidence or valid inherited Book. Allah says:
ومِنَ النَّاسِ مَن يُجَـدِلُ فِى اللَّهِ بِغَيْرِ عِلْمٍ وَلاَ هُدًى وَلاَ كِتَـبٍ مُّنِيرٍ
(And among men is he who disputes about Allah, without knowledge or guidance, or a Book giving light (from Allah).) (22:8) meaning clear and unambiguous.
وَإِذَا قِيلَ لَهُمْ
(And when it is said to them) means, to these people who dispute about the Oneness of Allah.
اتَّبِعُواْ مَآ أَنزَلَ اللَّهُ
("Follow that which Allah has sent down,") means, the pure Divine Laws that He has sent down to His Messengers,
قَالُواْ بَلْ نَتَّبِعُ مَا وَجَدْنَا عَلَيْهِ ءَابَآءَنَا
(they say: "Nay, we shall follow that which we found our fathers (following).") means, they have no other proof except the fact that they are following their forefathers. Allah says:
أَوَلَوْ كَانَ ءَابَاؤُهُمْ لاَ يَعْقِلُونَ شَيْئًا وَلاَ يَهْتَدُونَ
(Even though their fathers did not understand anything nor were they guided) (2:170) meaning, `what do you think, you who take the forefathers' deeds as proof, that they were misguided and you are following in their footsteps' Allah says:
أَوَلَوْ كَانَ الشَّيْطَـنُ يَدْعُوهُمْ إِلَى عَذَابِ السَّعِيرِ
(Even if Shaytan invites them to the torment of the Fire)
Tafsir Ibn Kathir (abridged, English).