Tafsir of Ar-Rum 30:6

(The promise of Allah) is an infinitive (masdar) that acts as a confirmation for the content of the preceding sentence, namely the words of the Exalted: "(they) will be overcome," and His saying: "the believers will rejoice." It is called a "self-confirmatory" infinitive because the preceding statement carries the meaning of a promise. Its governing agent is deleted obligatorily, as if it were said: "Allah the Exalted promised that, a promise..."

(He does not fail in His promise)—that is, any promise, whether it pertains to the worldly life or the Hereafter, because failing in a promise implies a deficiency in the one who makes it, which is impossible for Him, the Almighty and Majestic. The manifestation of the Majestic Name (Allah) in the place of a pronoun is for the sake of providing the legal reason for the ruling and for its glorification. The sentence is an elliptical clause establishing the meaning of the infinitive. It has also been suggested that it acts as a state (hal) for the infinitive, thus functioning like a qualified infinitive, as if the Exalted were saying: "Allah the Exalted promised a promise that is not failed."

(But most of the people do not know) that He, the Exalted, does not fail in His promise, due to their ignorance of His affairs, the Almighty and Majestic, and their lack of reflection upon what is necessary for His Majesty and what is impossible for Him, the Glorified. Or, it means they do not know what has preceded regarding His affairs, the Almighty and Majestic. It has also been said: they do not know anything, or they are not among the people of knowledge so that they might know that.