Maggi appositely cites Dorothy Sayers:
You would be ashamed to know as little about internal combustion as you know about Christian beliefs. I admit, you can practise Christianity without knowing much theology, just as you can drive a car without knowing much about internal combustion. But when something breaks down in the car, you go humbly to the man who understands the works; whereas if something goes wrong with religion, you merely throw the works away and tell the theologian he is a liar.
Now I have to track down the specific source of the quotation.
Ah! Paul has reproduced the whole letter (which I find less convincing, alas, than the excerpt) and duly notes the uncertainty regarding its provenance. And it turns out that Fred has more context, and a more satisfying rationale for the peremptory tone.
I tried recently (see http://exploringourmatrix.blogspot.com/2008/04/what-ordinary-people-think.html) but it seems that all the earliest quotations of the letter do not cite an original source where the letter itself can be found. Alas!
Hi AKMA, I think I found it in the volumes of her letters, somewhere around the fourth volume? but I also read various other bits of stuff about her at the same time, so I could be mistaken…