It sounds like it's sung from the point of view of a deposed ruler, possibly a French king after the Revolution. He's remembering a time where he was in power, and now that his reign has been overthrown, his focus is on the afterlife - St. Peter calling his name, and hearing the Roman choirs could be references to what sort of welcome he expects in heaven. I checked the link above - some people have also picked up on the Biblical references, and in that sense, it could be about someone like Jesus or John the Baptist (whose head was delivered on a silver plate), or any prophet who might have run into trouble with their government, and might be awaiting execution. That's if you take it all literally - it could be a metaphor for any situation where someone is reflecting on their life - someone who used to have a better job, or a better relationship, and is dreaming of a better life in the future. "Viva la Vida" could mean "Long live life" in addition to "Live the life". Revolutionary armies have been known to chant "Viva la Revolution", or "Long live the revolution!" That phrase is what makes me favor the French Revolution aspect over the Biblical prophet angle.