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.