A caution is in order concerning the opinions of the author. I know little about him, but he has been criticised for being anti-semetic.
Serbs have NEVER been “Crusaders” or “Templars”. Orthodox Christians have been targets and victims of the Crusaders—think Constantinople 1204 (4th Crusade) and the “Northern Crusade” of the Teutonic Knights against Orthodox Russia. Then there was the total slaughter in Jerusalem in the First Crusade—Jerusalem was majority Eastern Christian at the time.
Either Breivik’s Serbian (and Greek) associates were nominal Orthodox who were ignorant of their history, or Breivik is spinning a fantasy. I believe that the latter is the more likely of the two alternatives.
His 1,500+ page manifesto was written in remarkably good American English.