Tafsir of An-Nisa' 4:151

(Those): meaning those described by the aforementioned repulsive qualities.

(are the disbelievers): meaning those complete in disbelief, such that no consideration is given to what they claim and name faith in the first place.

(in truth): an infinitive acting as an emphatic for another, with its operative verb being elided, meaning: "That is true, in that their being complete in disbelief is a truth." It is also permitted that it be an adjective for the infinitive of "the disbelievers," meaning: "They are those who have disbelieved a disbelief in truth," that is, without doubt or suspicion; in this case, the operative verb is mentioned, and "truth" is used in the sense of a passive participle, not as the opposite of falsehood. This is why it is grammatically correct as an adjective, both structurally and semantically. As for the possibility of it being a circumstantial qualifier (hal), as Abu al-Baqa claimed, his view implies the verse is related to the Almighty’s saying, "O you who believe...", as a justification for it, with the sentences and verses intervening between the cause and the effect being either parenthetical or digressive upon closer inspection.

(And We have prepared for the disbelievers): meaning, for them. The explicit noun is used in place of the pronoun to highlight the heinous attribute of disbelief, which signals the reason for the punishment. It may also intend all disbelievers, and these [mentioned earlier] are included primarily.

(a humiliating punishment): a punishment that humiliates and abases them, as a recompense for the disbelief through which they expected honor.