I hope this title clarifies what some people have been asking. When Aļļaah promises or threatens, it means that He is telling us about what will happen in the future. This is not like human promises, which are mere words expressing commitment to do something in the future unseen. In other words, what He promises will happen, because His Speech is flawless and pertains to what He knows, not because He is obligated to fulfill promises. For this reason, saying that it is intrinsically impossible that His promise will not be fulfilled, is based on the fact that He has told us what will be in the future. It is not based on asserting any obligation.
I hope that clarifies it.
This current issue first came up with the Muˆtazilah or philosophers, who wanted to prove that there are acts that are impossible because of their intrinsic ugliness, i.e. they are intrinsically impossible because of intrinsic ugliness. They said that if you do not accept this, then you will end up saying that it is intrinsically possible that Aļļaah should lie, i.e. say one thing and do another.
The Asħˆariyys, knowing that this would be to insult Aļļaah, needed to show that lying is impossible, but without submitting to the claim that some acts are intrinsically ugly. This is because saying that possible acts can be intrinsically ugly, and therefore impossible, is to say that the intrinsically possible becomes impossible, and that is nonsense. Since it is nonsense, the real meaning of the Muˆtazilite saying, i.e. what it actually leads to saying, is that Aļļaah is obligated not to do certain acts, and to do others. This implies that Aļļaah needs to fulfill obligations, and that would be a kufr assertion.
The Asħˆariyys then, showed instead that you can say that lying is impossible without submitting to the claim that there are intrinsically ugly acts, because Aļļaah’s Speech is not created, and therefore is not something that falls in the possible category of things. Rather, it is an eternal and unchanging attribute that pertains to what His knowledge pertains to, by which He informs what He knows. If one said that it pertains to lies also, then one has attributed a flaw to Aļļaah’s speech, saying something untrue, and that is impossible, because it is imperfection.
Accordingly, the Asħˆariyys said that it is impossible that Aļļaah should have obligations, because that would imply a need to fulfill them, yet it is also impossible that He would not do what He promised, because His eternal Speech is flawless. So one says that Aļļaah definitely fulfills His promises and threats, because His Speech must be true. It is kufr to say that Aļļaah could do otherwise, because this is to say that His Speech could be false. This has nothing to do with ability, because that Aļļaah should say something untrue is intrinsically impossible.
Yet another way to explain this is to remember that Aļļaah is not in time. He is not something that changes and goes through stages of being. It is therefore impossible that He should have obligations, because being obligated by a promise means that the saying comes before the actions. Since neither Aļļaah’s actions, nor His Speech are in time, this sort of sequence does not befit Aļļaah, because He is not in time.