Those were the towns whose story We relate unto you. And there came indeed to them their Messengers with clear proofs, but they were not such who would believe in what they rejected before. Thus Allah does seal up the hearts of the disbelievers (101)And most of them We found not true to their covenant, but most of them We found indeed rebellious (102)

After narrating the stories of the people of Prophets Nuh, Hud, Salih, Lut and Shu'ayb, destroying the disbelievers, saving the believers, warning these nations by explaining the truth to them with the evidence sent in the words of His Messengers, may Allah's peace and blessings be on them all, Allah said;

تِلْكَ الْقُرَىٰ نَقُصُّ عَلَيْكَ

(Those were the towns that We relate to you) O Muhammad,

 مِنْ أَنبَائِهَا

(their story), and news,

وَلَقَدْ جَاءَتْهُمْ رُسُلُهُم بِالْبَيِّنَاتِ

(And there came indeed to them their Messengers with clear proofs,) and evidences of the truth of what they brought them. Allah said in other Ayah,

وَمَا كُنَّا مُعَذِّبِينَ حَتَّىٰ نَبْعَثَ رَسُولًا

(And We never punish until We have sent a Messenger (to give warning).)[17:15], and,

ذَٰلِكَ مِنْ أَنبَاءِ الْقُرَىٰ نَقُصُّهُ عَلَيْكَ مِنْهَا قَائِمٌ وَحَصِيدٌ - وَمَا ظَلَمْنَاهُمْ وَلَٰكِن ظَلَمُوا أَنفُسَهُمْ

(That is some of the news of the towns which We relate unto you; of them, some are standing, and some have been reaped. We wronged them not, but they wronged themselves.)[11:100-101] Allah said

فَمَا كَانُوا لِيُؤْمِنُوا بِمَا كَذَّبُوا مِن قَبْلُ

(but they were not such who would believe in what they had rejected before.) meaning they would not have later on believed in what the Messengers brought them, because they denied the truth when it first came to them (although they recognized it), according to the Tafsir of Ibn 'Atiyyah. This explanation is sound, and is supported by Allah's statement,

وَمَا يُشْعِرُكُمْ أَنَّهَا إِذَا جَاءَتْ لَا يُؤْمِنُونَ - وَنُقَلِّبُ أَفْئِدَتَهُمْ وَأَبْصَارَهُمْ كَمَا لَمْ يُؤْمِنُوا بِهِ أَوَّلَ مَرَّةٍ

(And what will make you perceive that if it came, they will not believe? And We shall turn their hearts and their eyes away (from guidance), as they refused to believe therein for the first time.)[6:109-110]

This is why Allah said here,

كَذَٰلِكَ يَطْبَعُ اللَّهُ عَلَىٰ قُلُوبِ الْكَافِرِينَ - وَمَا وَجَدْنَا لِأَكْثَرِهِم

(Thus Allah does seal up the hearts of the disbelievers. And most of them We found not...) meaning, We did not find most of the previous nations,

مِّنْ عَهْدٍ وَإِن وَجَدْنَا أَكْثَرَهُمْ لَفَاسِقِينَ

(true to their covenant, but most of them We found to indeed be rebellious.)

This Ayah means, We found most of them to be rebellious, deviating away from obedience and compliance.

The covenant mentioned here is the Fitrah that Allah instilled in them while still in their fathers' loins, and taking their covenant, that He is their Lord, King, and that there is no deity worthy of worship except Him,. They affirmed this covenant and testified against themselves to this fact. However, they defied this covenant, threw it behind their backs and worshipped others besides Allah, having no proof or plea, nor support from rationality or by divine law. Surely, the pure Fitrah defies these actions, while all the honorable Messengers, from beginning to end, forbade them. Muslim collected the Hadith,

يَقُولُ اللهُ تَعَالَى إِنِّي خَلَقْتُ عِبَادِي حُنَفَاءَ فَجَاءَتْهُمُ الشَّيَاطِينُ فَاجْتَالَتْهُمْ عَنْ دِينِهِمْ وَحَرَّمَتْ عَلَيْهِمْ مَا أَحْلَلْتُ لَهُم

(Allah said, "I created My servants Hunafa' (monotheists), but the devils came to them and deviated them from their religion and prohibited them what I allowed them.")

It is recorded in the Two Sahihs,

كُلُّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ فَأَبَوَاهُ يُهَوِّدَانِهِ وَيُنَصِّرَانِهِ وَيُمَجِّسَانِهِ

(Every child is born upon the Fitrah, it is only his parents who turn him into a Jew, a Christian or a Zoroastrian.)