The word catches the notion of separateness, of being different. In the category called “gods,” the God and Father of our Lord Jesus Christ has no companion or competition, for he is the only God who is real, who exists. That is even more so on the matter of sending his Son to become human in a fallen world; no other god has ever done that. So he is different, unique, in a class by himself, “holy.” Children of this Father need to conform themselves to his uniqueness, his holiness, and that means they be out of step (“not be conformed”) with average expectations in human society (the unborn mainstream) in order to be in step with the God who is different/unique.
15 but as he who called you is holy, you also be holy in all your conduct,