يَا أَيُّهَا الْكَافِرُونَ
O disbelievers! (109:1)
Yā marks the vocative; ayyu is the vocative noun; hā alerts the audience; and al-kāfirūn specifies or explains the addressee under recognized grammatical analyses. Explicit naming establishes the group being addressed, rather than leaving the proclamation attached to an unspecified audience.
Rhetorically, the call demands attention before sustained negation. One must not infer from this grammatical form an unsupported personal judgment about every individual across all times; the immediate function is to frame the surah's stated declaration.