You must find all 33 Hospitalier flags in Acre.
for an awesome guide to the Hospitalier flags (as well as all other flags) and the Templars.
Please give full credit to FiReStOrM^
for his awesome work and dedication to making this incredible resource.
In case his page goes down again,I have left videos of all Hospitalier Flags in Acre, credit to youtube user TheGameStory.