This song is about the crucifiction of Jesus for the following reasons:.
Saint Peter wont call my name- Refers to the disciple that would not admit to knowing Jesus.
Jeruselam bells and Roman cavelry: Jesus was crucified in Jeruselam by the romans.
Seas would rise.: Refers to the parting of the storm, parting of the red sea, walking on water etc.
Castles on Pillers of salt/sand: Refers to a parable told by Jesus where two houses were built one on rocks and one on sand.
All the king talk: Jesus was the king of the jews.