Learn Italian: Un vero idealista vive per il suo idealismo. - A true idealist lives for their idealism.

Nouns ending in the suffix -ista (for adherents or professions) are identical for both masculine and feminine in the singular. We know this person is male because of un vero. If it were a female idealist, it would be un'idealista vera. In the plural, the endings split: -isti for men and -iste for women.

Are nouns ending in -ismo, like idealismo, always masculine?

Yes. The suffix -ismo, which forms nouns denoting an ideology, movement, or characteristic, creates exclusively masculine nouns. You will always use masculine articles and adjectives with them, such as il suo in this sentence.

The English translation says "their idealism", but Italian uses il suo. Why not il loro?

In English, "their" is used here as a gender-neutral singular pronoun for "a true idealist". Italian doesn't use the plural loro this way. Instead, you use the third-person singular possessive suo. Remember that suo changes its ending to match the noun being owned (the masculine idealismo), not the person who owns it!

Would it mean the same thing if I wrote un idealista vero instead of un vero idealista?