Here's the song, with a midi file.

Blasphemous or not, it seems to me pretty clear that the song is talking about storming the gates of Heaven, if need be.

After all the song (though it was the work of a parson (Parson Hawker, Vicar of Morwenstowe, in 1825) wasn't writtem until well over onme hundred yesars after Bishop Trelawny's fairly brief spell in jail for his opposition to King James's attempt to stop the persecution of Catholics.