This appears to be a very old myth, dating as far back as the Civil War, when small Bibles were commonly distributed to soldiers and often kept in the front pocket of uniforms as a sort of good-luck charm.

 

The myth has become so popular in both Pentecostal and non-Pentecostal circles that it was the subject of an episode on the popular Discovery Channel show Mythbusters, where is was conclusively proven that a Bible, even a hardcover edition, is completely unable to stop a bullet.

 

As a side note:  similar stories of bulletproof pastors and/or missionaries abound in Pentecostal churches, including many versions that claim that a pastor was protected only by the Holy Spirit and deflected bullets off his skin or caught them in his teeth.

 

Pentecostal pastors are often viewed by their congregation as having supernatural protection and superhuman powers.  All of these myths serve to increase the congregation's awe of church authority and convince them that their pastor is a supernatural person with a special anointing, to the extent that bullets will not even harm him.

 

For a brief commentary on the Mythbusters, including their investigation of the bulletproof Bible myth, click the link below: