“Make the enemy live up to their own book of rules. You can kill them with this, for they can no more obey their own rules than the Christian church can live up to Christianity.”

Encyclical Fides et Ratio, 14 September 1998
Source: www.vatican.va http://www.vatican.va/holy_father/john_paul_ii/encyclicals/documents/hf_jp-ii_enc_14091998_fides-et-ratio_en.html

The Divine Commodity: Discovering A Faith Beyond Consumer Christianity (2009, Zondervan)

"Our .NET Strategy" http://www.joelonsoftware.com/articles/Our.NetStrategy.html

Though Keillor has been quoted on the internet and in print as having made this or a similar remark, such expressions have been made by others, and may have originated with Billy Sunday, who is quoted as having said "Going to church on Sunday does not make you a Christian any more than going into a garage makes you an automobile!" in Press, Radio, Television, Periodicals, Public Relations, and Advertising, As Seen through Institutes and Special Occasions of the Henry W. Grady School of Journalism (1967) edited by John Eldridge Drewry.
Disputed
Variant: Going to church no more makes you a Christian than standing in a garage makes you a car.

Though Keillor has been quoted on the internet and in print as having made this or a similar remark, such expressions have been made by others, and may have originated with Billy Sunday, who is quoted as having said "Going to church on Sunday does not make you a Christian any more than going into a garage makes you an automobile!" in Press, Radio, Television, Periodicals, Public Relations, and Advertising, As Seen through Institutes and Special Occasions of the Henry W. Grady School of Journalism (1967) edited by John Eldridge Drewry.
Disputed

Source: The White Rose (1985), Chapter 28, “To the Barrowland” (p. 576)

Source: The Invention of Hugo Cabret & Official 'Hugo' Movie Companion