I’m reading a new biography (2006) of Mark Twain by Ron Powers and I’ve read this passage about 6 times and I may be missing something, so…
The passage on page 49 seems to suggest the invention of the invective of “Jesus H. Christ.” (I’m typing here and it is exceedingly complex, so I hope there are not too many mistakes).
This visit produced what might be called the Gospel according to Wales [typesetter, 17 year-old Wales McCormick]. In one version of the story, Twain insisted that when Campbell [Alexander, son of Thomas Campbell, the founder of the Campellites and co-founder of the Disciples of Christ] stopped by Ament’s shop with the sermon [that several had requested that this shop reprint], he overheard McCormick exclaim, “Great God!” The preacher took the boy aside and admonish him that “Great God!” was blasphemy, and that “Great Scott!” would be one example of an acceptable substitute. McCormick apparently took this to heart: while correcting the proof sheet of the sermon, he dutifully changed Campbell’s own pious use of “Great God” to “Great Scott.” Taken with the spirit, he [McCormick] amended “Father, Son and Holy Ghost” to “Father, Son & Great Caesar’s Ghost,” and then improved even that bit of euphemism to “Father, Son and Co.” [The Autobiography of Mark Twain: edited by Charles Neider: New York: Harper & Row; 1959: pp. 60-61]
Wales’s moment of divine reckoning approached when he removed the full name “Jesus Christ” from a line in the sermon to create more space, and substituted “J.C.” For some reason, this infuriated Campbell as he read the proof sheet; he strode back to the print shop and commanded McCormick: “So long as you live, don’t you ever diminish the Savior’s name again. Put it all in.” McCormick took this too heart: the revised line came out, “Jesus H. Christ.”
I’m not sure where the “H” comes from and it all sounds like a folk etymology. But I don’t know. It is searchable on Amazon by the above link for those who care to do it. Search for Jesus H Christ
NB: the “Jesus H Christ” quote is not in any of the notes cited as far as I can tell.
Edit: For what it’s worth: Wikipedia has:
The expression dates to at least the late nineteenth century (although according to Mark Twain it was already old in 1850), and likely originates with the ancient Christian three-letter symbolism IHS (the Christogram).
So maybe it’s not the invention, but an early reporting.