That's not really true. The insanity plea has existed in the common law at least since the 13th century, though like many affirmative defences including self-defense, it was a reason for pardon, and not a reason for acquittal. The M'Naughton rule simply codified existing practice in the English courts, and since it was a good rule, was adopting in American courts as well, mostly. I do agree that the notion that this is all "christian" is bogus!