Hey guys, just stating my opinion on the meaning.
Even though the song may have religious references, I don't think it is talking about any religious character. It would be more precise if it did.
I think the band alluded to monarchy or kings from the beginning: "used to rule the world/ seas would rise when i gave the word";"sweep the streets i used to own";"now the old king is dead-long live the king". For me it is quite clear that they are talking about a monarch, although vaguely. Perhaps the reference to control of the seas has to do with British monarchs.
Then, "one minute i held the key/ next the walls were closed on me" could indicate that he/she who had it all, lost it all.
In the refrain, "be my mirror my sword and shield/be my missionaries in a foreign field" again could indicate the power of a king, who employs others to do his bidding.
"Once you know there was never, never an honest word/that was when i ruled the world", means that the character realises that when he had it all, it also meant nothing, life was hollow.
Finally, "I know Saint Peter will call my name/never an honest word/ but that was when i ruled the world", alludes to acceptance into heaven (saint Peter will call my name), despite there never being an honest word, since that was BEFORE.
Generally, i think this song means that even when you think you have it all, life is not so simple, and what gives you power or standing, may not be meaningful (no honest word). Only when you realise this and change your ways, can you achieve happiness (admitted into heaven). Perhaps the band was trying to say with "Viva la Vida" to live life without any misguided preconceptions about power and wealth. Keep it simple. Or something like that.
Anyway, that is just my interpretation. I love this song, because it ties music and poetry: great piece of work!