Tafsir of Al-Fath 48:13

{And whoever does not believe...}

The [explicit] mention of {for the disbelievers} stands in place of [the pronoun] "for them," to signal that whoever does not combine the two beliefs—belief in Allah and belief in His Messenger—is a disbeliever.

He used the indefinite form for {a Blaze} (saʿīran) because it is a specific [type of] fire, just as He used the indefinite form for {a Fire that rages} (Al-Layl: 14).

{And to Allah belongs the dominion of the heavens and the earth. He forgives whom He wills and punishes whom He wills. And Allah is Ever-Forgiving, Most Merciful.}