“Going to church on Sunday does not make you a Christian any more than going into a garage makes you an automobile!”

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

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.

Source: Kiss of the Night

279
The Reasonableness of Christianity (1695)

Source: Rules for Radicals: A Practical Primer for Realistic Radicals (1971), p. 128

Source: Vorkosigan Saga, Memory (1996)
Context: "You go on. You just go on. There's nothing more to it, and there's no trick to make it easier. You just go on."
"And what do you find on the other side? When you go on?"
"Your life again. What else?"
"Is that a promise?"
"It's an inevitability. No trick. No choice. You just go on."

Source: From "Why Gays (as a Group) are Morally Superior to Christians (as a Group)" in The Hauerwas Reader https://www.academia.edu/6641759/Introduction_to_The_Hauerwas_Reader_2001_ (2001) eds. John Berkman and Michael Cartwright