I wonder if this would work? This is old but hopefully it will: http://ourl.ca/14486 It's written in Lua so you don't need Ndless software to use it.

The bible is quite a lot of pages... I wonder if it fits?

Not sure about the >1 day but yeah the file size will be big.

Well, the Nspire is a whole different system than the Z80 calcs, so I have only done BASIC for the most part on it.

LZ compression followed by statistical compression would probably get the best results. LZ compression would compress any repeated words or phrases to 4 bytes (maybe 5 depending on how large the file actually is). This might not seem good, but the bytes are just a pointer to the existing phrase. For example, the following:

gird yourselves, and ye shall be broken in pieces; gird yourselves, and ye shall be broken in pieces.

Since it is less than 256 bytes would be compressed to something like the following:
[0+49]gird yourselves, and ye shall be broken in pieces[0+2]; [128+0][0+1].
Everything in [] is a single byte. So the original phrase would have been 101 bytes, but the encoded string would have been 1+49+1+2+1+1+1=56 bytes. However, in the much larger file of a book, words like "ye", "be", and "yourselves" might be encoded in a single byte (like BASIC tokenization as codebender described) so the compression would be significantly better.

If you have a compressed copy on your computer, chances are software can be easily written to decompress on your calc.

Please don't use nPDF, it's too buggy. I need to improve it, but never get time.

You could also use mViewer GX, which can convert PDFs, but it converts to images so I doubt a full bible will ever come close to fitting on calc.

you could use ntxt.