We talk a lot about what kind of pen or pencil to write in your Bible with, so ... don't try this at home. It takes a brave soul to try a fountain pen in a Bible. As I mention at my other blog, I'm a bit obsessed with fountain pens, archaic though they may seem:
When I do write in a Bible, I tend to use a Pigma Micron on a ballpoint, avoiding anything that can apply "wet" ink. In theory, though, using a fountain pen in a Bible shouldn't be a problem. After all, the Allan's Journal uses Bible paper and holds up just fine to FP ink:
On the left you can see various examples of ink in use (as well as a sad testament to what happens to one's cursive when one starts writing in block letters during junior high). On the left, note that while you can see the writing through the reverse of the page (just as you would printing in a Bible), none of the ink has bled through. Now, if you're inclined to try this, my recommendation is that you use a Fine (F) or even Extra Fine (EF) nib. These put less ink on the page and give you a cleaner, finer line, allowing you to write smaller, in proportion to the small print in the typical Bible.
For more thoughts on fountain pens, check out "The Craftsman's Pleasure in His Tools."