Well, there are six sons of Abraham, but Abraham was an important person in Judaism. Judaism was the first Abrahamic religion and from it came Christianity, and to some extent, Islam. Also, there are seven letters in both Abraham and Judaism and due to Abraham's big connection to Judaism this must be true: Abraham = Judaism. If we look at the title that way then we can call it Crusader Kings II: The Sons of Judaism. If we're thinking of sons as if it's a family, then this would mean that it's referring to religions that had Judaism as a base. If you look back at sentence two you can see that two of these six religions are Christianity and Islam, but what are the other four?