It is important to understand the goals of the PLO to understand Christian Arabs in the ISM/PSM:
The PLO is a "revolutionary movement" with many factions. Arafat was actually a proxy for the Soviet Union during the Cold War so communism has always played an important role in the PLO.
Some PLO factions are communists, some democratic, some theocratic (like Hamas), some secular (like Fatah). It is important to understand Middle East tribalism, as well. All factions of the PLO are pan-Arabists after the late Gamel Nasser's desire to create a United Arab Republic in the 1950's among all Arab nations.
Most Christian Arabs are in fact sympathetic to Israel. However, many who have grown up in a pan-Arab culture with hatred of Jews almost since birth adopt the major tribal view of the need to destroy Israel. Also, Christian Arabs brought up in an alternative culture were in a position to learn about communism to become communists whereas Muslims who become apostates face murder or family alienation. Hence, many of the Christian Arabs who run around for the militant Islamists are more "Christian" by ancestry than practice, and are in reality communists who use their Christian background as a weapon against Israel and "the Jews." The PFLP for example, is composed mainly of Christian Palestinian Arabs who are communists in reality and pan-Arabists like Mazen Qumsiyeh. They abandoned Jesus Christ for a new god of Marxism, or claim Jesus was a Palestinian and not a Jew to deny any Jewish connection to the land of Israel.