Tafsir of Al-Fath 48:13

(And whoever does not believe in Allah and His Messenger...) – The speech here is initiated from His side, Exalted be His Majesty, and is not included within the recited discourse. It is a confirmation of their ruin and an explanation of its manner. That is: whoever does not believe in Allah, the Exalted, and His Messenger, may Allah bless him and grant him peace, as is the habit of these who remained behind, (then indeed, We have prepared) – meaning, We have readied (for the disbelievers a blazing fire).

(A fire, stoked, kindled, and flaming). The apparent phrasing would have been "for them" [i.e., for those who remained behind], but it shifted from that to what was mentioned to signal that whoever does not combine faith in Allah, the Exalted, and His Messenger, upon him be blessings and peace, is a disbeliever, and that he is deserving of the blazing fire due to his disbelief, by virtue of the attribution to the derived noun (the disbelievers).

The use of the indefinite form for "a blazing fire" (sa'iran) is for the purpose of intensification, as it indicates that its nature is beyond comprehension and its essence cannot be fully grasped. It is also said: It is because it is a specific fire, and the indefiniteness is for classification. The word "whoever" (man) may be relative or conditional, and the referent (the pronoun/link) in the predicate or the answer to the condition is the apparent noun acting in place of the implied pronoun.