From Halopedia, the Halo wiki
Believe is a Microsoft marketing campaign for Halo 3 produced by New Deal Studios. It features the word "Believe" as a tagline, and appears to be honoring the legacy of SPARTAN-117, citing him as a hero of the United Nations Space Command whose brave actions inspired other servicemen during the Human-Covenant War.
It includes handouts, live-action TV commercials, magazine page advertisements, and Xbox live marketplace ads.
(Note the Websites are defunct now. Anything that can be found is either on its relevant page or on this page)
Jack Courage site