Allah condemns the idolators for worshipping others instead of Allah with no evidence or proof for doing so.

﴿أَمْ ءَاتَيْنَـهُمْ كِتَـباً مِّن قَبْلِهِ﴾

(Or have We given them any Book before this) means, before their idolatry.

﴿فَهُم بِهِ مُسْتَمْسِكُونَ﴾

(to which they are holding fast) means, with regard to what they are doing. This is not the case. This is like the Ayah:

﴿أَمْ أَنزَلْنَا عَلَيْهِمْ سُلْطَـناً فَهُوَ يَتَكَلَّمُ بِمَا كَانُواْ بِهِ يُشْرِكُونَ ﴾

(Or have We revealed to them an authority (a Scripture), which speaks of that which they have been associating with Him) (30:35) meaning, that did not happen. Then Allah says:

﴿بَلْ قَالُواْ إِنَّا وَجَدْنَآ ءَابَآءَنَا عَلَى أُمَّةٍ وَإِنَّا عَلَى ءَاثَـرِهِم مُّهْتَدُونَ ﴾

(Nay! They say: "we found our fathers following a certain way and religion (Ummah), and we guide ourselves by their footsteps.'') meaning, they have no grounds for their belief in idolatry apart from the fact that they are imitating their fathers and forefathers who were following a certain Ummah or way, i.e., religion. The word Ummah is used in a similar manner elsewhere, where Allah says:

﴿إِنَّ هَـذِهِ أُمَّتُكُمْ أُمَّةً وَاحِدَةً﴾

(And verily, this your religion (Ummah) is one religion) (23:52), and they said;

﴿وَإِنَّا عَلَى ءَاثَـرِهِم﴾

(and we by their footsteps) means, behind them

﴿مُّهْتَدُونَ﴾

(We guide ourselves) This is their claim with no evidence. Then Allah points out that what these people say was already said by those who were like them among the nations of the past who disbelieved in the Messengers. Their hearts and their words are similar.

﴿كَذَلِكَ مَآ أَتَى الَّذِينَ مِن قَبْلِهِمْ مِّن رَّسُولٍ إِلاَّ قَالُواْ سَـحِرٌ أَوْ مَجْنُونٌ - أَتَوَاصَوْاْ بِهِ بَلْ هُمْ قَوْمٌ طَـغُونَ ﴾

(Likewise, no Messenger came to those before them but they said: "A sorcerer or a madman!'' Have they transmitted this saying to these Nay, they are themselves a people transgressing beyond bounds!) (51:52-53) And Allah says here:

﴿وَكَذَلِكَ مَآ أَرْسَلْنَا مِن قَبْلِكَ فِى قَرْيَةٍ مِّن نَّذِيرٍ إِلاَّ قَالَ مُتْرَفُوهَآ إِنَّا وَجَدْنَآ ءَابَآءَنَا عَلَى أُمَّةٍ وَإِنَّا عَلَى ءَاثَـرِهِم مُّقْتَدُونَ ﴾

(And similarly, We sent not a warner before you to any town (people) but the luxurious ones among them said: "We found our fathers following a certain way and religion, and we will indeed follow their footsteps.'') Then He says:

﴿قُلْ﴾

(Say) -- `O Muhammad, to these idolators --'

﴿أَوَلَوْ جِئْتُكُمْ بِأَهْدَى مِمَّا وَجَدتُّمْ عَلَيْهِ ءَابَآءَكُمْ قَالُواْ إِنَّا بِمَآ أُرْسِلْتُمْ بِهِ كَـفِرُونَ﴾

("Even if I bring you better guidance than that which you found your fathers following'' They said: "Verily, we disbelieve in that with which you have been sent.'') `Even if they were convinced of the truth of what you have brought to them, they will not follow it, because of their evil intentions and their arrogance towards the truth and its people.'

﴿فَانتَقَمْنَا مِنْهُمْ﴾

(So We took revenge on them) means, on the disbelieving nations, by inflicting various kinds of punishments, as Allah has described in the stories of those nations.

﴿فَانظُرْ كَيْفَ كَانَ عَـقِبَةُ الْمُكَذِّبِينَ﴾

(then see what was the end of those who denied) means, see what became of them, how they were destroyed and how Allah saved the believers.

﴿وَإِذْ قَالَ إِبْرَهِيمُ لاًّبِيهِ وَقَوْمِهِ إِنَّنِى بَرَآءٌ مِّمَّا تَعْبُدُونَ - إِلاَّ الَّذِى فَطَرَنِى فَإِنَّهُ سَيَهْدِينِ - وَجَعَلَهَا كَلِمَةً بَـقِيَةً فِى عَقِبِهِ لَعَلَّهُمْ يَرْجِعُونَ - بَلْ مَتَّعْتُ هَـؤُلاَءِ وَءَابَآءَهُمْ حَتَّى جَآءَهُمُ الْحَقُّ وَرَسُولٌ مُّبِينٌ - وَلَمَّا جَآءَهُمُ الْحَقُّ قَالُواْ هَـذَا سِحْرٌ وَإِنَّا بِهِ كَـفِرُونَ - وَقَالُواْ لَوْلاَ نُزِّلَ هَـذَا الْقُرْءَانُ عَلَى رَجُلٍ مِّنَ الْقَرْيَتَيْنِ عَظِيمٍ - أَهُمْ يَقْسِمُونَ رَحْمَةَ رَبِّكَ نَحْنُ قَسَمْنَا بَيْنَهُمْ مَّعِيشَتَهُمْ فِى الْحَيَوةِ الدُّنْيَا وَرَفَعْنَا بَعْضَهُمْ فَوْقَ بَعْضٍ دَرَجَـتٍ لِّيَتَّخِذَ بَعْضُهُم بَعْضاً سُخْرِيّاً وَرَحْمَةُ رَبِّكَ خَيْرٌ مِّمَّا يَجْمَعُونَ - وَلَوْلاَ أَن يَكُونَ النَّاسُ أُمَّةً وَحِدَةً لَّجَعَلْنَا لِمَن يَكْفُرُ بِالرَّحْمَـنِ لِبُيُوتِهِمْ سُقُفاً مِّن فِضَّةٍ وَمَعَارِجَ عَلَيْهَا يَظْهَرُونَ - وَلِبُيُوتِهِمْ أَبْوَباً وَسُرُراً عَلَيْهَا يَتَّكِئُونَ - وَزُخْرُفاً وَإِن كُلُّ ذَلِكَ لَمَّا مَتَـعُ الْحَيَوةِ الدُّنْيَا وَالاٌّخِرَةُ عِندَ رَبِّكَ لِلْمُتَّقِينَ ﴾

(26. And (remember) when Ibrahim said to his father and his people: "Verily, I am innocent of what you worship,'') (27. "Except Him Who created me; and verily, He will guide me.'') (28. And he made it a word lasting among his offspring, that they may turn back.) (29. Nay, but I gave to these and their fathers to enjoy, till there came to them the truth, and a Messenger making things clear.) (30. And when the truth came to them, they said: "This is magic, and we disbelieve therein.'') (31. And they say: "Why is not this Qur'an sent down to some great man of the two towns'') (32. Is it they who would portion out the mercy of your Lord It is We Who portion out between them their livelihood in this world, and We raised some of them above others in ranks, so that some may employ others in their work. But the mercy of your Lord is better than what they amass.) (33. And were it not that mankind would have become of one community, We would have provided for those who disbelieve in the Most Gracious, silver roofs for their houses, and elevators whereby they ascend,) (34. And for their houses, doors, and thrones on which they could recline,) (35. And adornments of gold. Yet all this would have been nothing but an enjoyment of this world. And the Hereafter with your Lord is for those who have Taqwa.)