You must kill all 60 Templars found throughout the world of Assassins Creed.http://www.assassinscreed-maps.com/
Please visit this website for an INCREDIBLE guide to the Templars, one of the best online guides i've ever seen, be sure and drop him a line on his great work.
Note: The Templars are the green dots found on the flag maps.
Note #2: Often one or a few of the templars will glitch and not be there.
There are a few things you can try to make him appear, leave the area (preferably the city you're in) and then come back and approach where he is again (maybe even from a different direction).
Also you can try jumping to your death nearby the templar and when you respawn he should be in his spot. Thanks to x1001x Puppys
for this tip, he did this and the glitched templar was there
I have left videos of the templar locations in case of the website going down, credit to youtube user TheGameStory