Sequence
Earlier, in ; يَقُولُونَ ,(they say - 75), there was a refutation of the claim made, by the people of the Book. Onwards from there, in verses 76-77, the same refutation has been re-asserted and the merit of fulfilling a commitment as well as the condemnation for its breach have been clarified.
Commentary
Ahd عھد (pledge or covenant) is what gets settled between parties concerned after mutual discussions and by which both of them have to abide. Contrary to this is وعدہ wa'dah or promise which issues forth from a single side, that is, عہد ` ahd (pledge or covenant) is bilateral while wa'dah (promise) is unilateral.
That commitments should be fulfilled has been stressed in the Qur'an and Sunnah time and again. For instance, right here in verse 77 cited above, five warnings have been given to those who break their solemn pledge:
1. They will have no share in the blessings of the heaven. In a hadith, the Holy Prophet ﷺ has said that one who usurps the right of a Muslim under false oath makes the fire of Hell compulsory for himself. The narrator of the hadith asked if the Fire will become compulsory even if this concerned something very insignificant? In reply, he said: Even if this be the green bough of a tree. (Muslim vide Mazhari)
2. Allah Almighty will not speak to them with glad tidings.
3. Allah Almighty will not look at them mercifully on the Day of Doom.
4. Allah Almighty will not forgive them their sins since they wasted away the rights of a servant of Allah through breach of trust, and Allah will not forgive what a human being owes to another human being. In Islamic terminology, this is known as the hagq al-'abd or the right of a servant of Allah.
And a grievous punishment shall await them.