وَمَا وَجَدْنَا لَاكْثَرِهِم
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 وَيُمَجِّسَانِه
Every child is born upon the Fitrah, it is only his parents who turn him into a Jew, a Christian or a Zoroastrian