What does Tom keep in his pocket for safety from the devil?

From the text:

Tom had a lurking dread that the devil, after all, would have his due. That he might not be taken unawares, therefore, it is said he always carried a small Bible in his coat-pocket. He had also a great folio Bible on his counting-house desk, and would frequently be found reading it when people called on business; on such occasions he would lay his green spectacles in the book, to mark the place, while he turned round to drive some usurious bargain.