It is as sure as John Gill is in the bookseller's shop.

In the Memoir of John Gill’s life that appears in the preface to A Body of Doctrinal and Practical Divinity (Baptist Standard Bearer, reprint, 2007), this anecdote is told about the boy Gill’s love of reading and books and his frequent visits to the local bookshop:

This shop he so regularly attended, for the sake of consulting different Authors, that is became an usual asservation with the people in the neighborhood, when they desired to express a certainty,--“It is as sure, as that John Gill is in the bookseller’s shop.” And as the studious disposition attended him though life, so did nearly the same remark; those who knew him usually employing this mode of affirmation,--“As surely as Dr. Gill is in his study.” (p. xviii).