Tafsir of Al-Furqan 25:56

"And We have not sent you" — in any state of affairs — "except as" — in the state of being — "a bearer of glad tidings" — to the believers — "and a warner" — that is, one who warns, with the warning intensified, to the disbelievers.

Because the warning is specific to them, and the discourse concerns them, and it serves to indicate the extent of their persistence in the error they are in, the superlative form is restricted to it [the warning]. It has been said that the intensification refers to the multitude of those being warned, for the disbelievers in every era are more numerous than the believers. Some have considered their multitude by including the disobedient among the believers within them; that is, a warner to the disobedient, whether they be believers or disbelievers.

However, the context demands specificity to the disbelievers, as is not hidden. The intended meaning is: "We have not sent you except as a bearer of glad tidings to the believers and a warner to the disbelievers, so do not grieve over their lack of belief."