significance of the title the sympathizer

This story follows an unnamed undercover international spy , a sympathizer,who departs Saigon, Vietnam in 1975 to live in Los Angeles as an agent to the Communists. He tells us about himself, that he is half Vietnamese and half French, and that his father was a Catholic priest who broke his vow of celibacy to impregnate his mother. Although he reports to the Communists in Vietnam, he is friendly with the CIA and with other Vietnamese people who are not Communists.