From Halopedia, the Halo wiki
Faithless is the fifth track in Halo 4: Original Soundtrack.
The track opens with distorted orchestral strings and percussion building up a melody, joined by guitar and horns as the melody. Strings takes over with a tense melody, then gradually simmer down. They build up with a suspenseful melody, later joined by percussion, and gradually fading to an end.
The track can be heard:
The track was initially composed by Neil Davidge, with additions from Andrew Morgan, who added counter parts to Davidge's original orchestration. Jeremy Wheatley performed audio mixing to the track, and, according to Davidge, it only took one attempt to finalize it.