Luke Sarsfield was murdered by Gavriel Larca to trigger his Immortality. Larca then told him that he was God, and that he was chosen to defeat Satan for him, Satan being, in this case, Duncan MacLeod.
Together with Larca's other disciples, Derek Worth and Enrique Grimaldo, they attacked Duncan MacLeod near a cemetery. When they pursued him into the cemetery, MacLeod tried to get them to understand they could not fight on holy ground, only then did Larca appear and confirm even Satan could claim that refuge.
They returned to an abandoned church, where Larca told Luke and the others about his plans to take Satan's (Duncan's) head. Some time later they were confronted by Reverend Thomas Bell, a friend of Derek.
Larca commanded Luke to kill Bell, but he wouldn't. Shortly thereafter, Larca beheaded Luke for his disobedience, and took his Quickening.
Luke never became aware of what he really was, not an angel but an immortal, not different to MacLeod or Larca, to whom he lost his head.