What seems to make sense
I don't think this is referring to Jesus Christ--I think it's referring to Constantine, the first "Christian" Roman emperor.
Here's why: Constantine had a desire to visit Jerusalem and be baptized in the River Jordan, but put it off in order to be absolved from as much sin as possible. He didn't make it.
That accounts for a large portion of the chorus:
"I hear Jerusalem bells a-ringing"
He wanted to visit the Jerusalem area--it was his dying wish
"Roman cavalry choirs are singing"
He had been a great military leader--and most of his troops remained loyal to him for life.
"Be my mirror, my sword and shield
My missionaries in a foreign field"
Constantine proclaimed religious tolerance, and founded the "Christian Byzantine Empire," which later split into the Roman Catholic and Eastern Orthodox churches. He did a lot to spread Christianity.
"For some reason I can't explain
I know St Peter won't call my name
Never an honest word
But that was when I ruled the world"
He wasn't baptized as he wanted and as he felt he needed. He also lived a very violent life full of military campaigns, and even in times of peace he had to play the role of a traditional roman politician (which meant appease everyone by lying or get killed).
"It was a wicked and wild wind
Blew down the doors to let me in
Shattered windows and the sound of drums
People couldn't believe what I'd become"
The rulers before Constantine had a policy of religious oppression towards Christianity (this is where modern Christians get the fish symbol, as the ancient Christians used that to identify fellow members of the faith without revealing themselves to the Roman regime). Being Christian before Constantine meant you either rebelled against the government openly, or lived a very secretive life. Hence, from Constantine's view, when the Roman people refused to acknowledge Diocletian's rule and showed seeds of rebellion--his rise to power was made possible by wicked people and the ever shifting political landscape of Rome.
The last line probably refers to Constantine's abolishment of the Tetrachy (a four-sided military governing regime).
Constantine also put down two very large rebellions and three civil wars, hence:
"Revolutionaries wait
For my head on a silver plate"
Not to mention the name of the song is Latin--the language spoken at Constantine's time (Christ would have spoken Aramaic, Hebrew, or Greek).
So, there you go, I don't rightly know how to explain the first lines, other than a "could be" Constantine's last thoughts, since he died in a suburb of Rome before he could make it back to the palace.