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.