I suspect that you are making the common error of confusing morality with
religion. I would think that Paine's statement in the biography that Clemens
did NOT undergo any deathbed conversion should be enough to require us to
take the man at his word with regard to his beliefs.

Religion certainly holds no monopoly on expressions of moral outrage or
other worthy sentiments. Only by defining religion almost out of
existence can one bring the author of Letters From the Earth "into the
fold."

That's just a non-scholarly, non-deconstructionist opinion from someone
who has read Twain extensively and repeatedly, and who has found no
evidence that he was religious from the time that the told Livy that
he had given it his best shot (for her sake), but just couldn't
believe.

Peggy A Dolan