Why is it der Nationalist?

Nationalist ends in -ist, and nouns ending in -ist are masculine: in a German dictionary, 97% of them take der.

All nouns ending in -ist
Sense 1
der

Meaning

[1] Pendant/representative of the nationalism someone someone who represents the interests of their own nation over everyone else

[1] Anhänger/Vertreter des Nationalismus; jemand, der die Interessen der eigenen Nation über alle anderen stellt

Examples

[1] "My grandparents and my great -grandmother had to leave their homes in 1930 when more and more Muslims were victims of the violence of Serbian nationalists"

[1] „Meine Großeltern und meine Urgroßmutter hatten schon 1930 ihre Heimat verlassen müssen, als immer mehr Muslime Opfer der Gewalt serbischer Nationalisten wurden.“

Declension

1 Singular Plural
Nominativ der Nationalist die Nationalisten
Genitiv des Nationalisten der Nationalisten
Dativ dem Nationalisten den Nationalisten
Akkusativ den Nationalisten die Nationalisten

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary