Dr. Schwartz attempts to demonstrate the influence of Robert G. Ingersoll’s works on Samuel Clemens, writing as Mark Twain. While Clemens never proclaimed his beliefs or lack thereof to the world, his treatment of religion in his works indicates that he was a freethinker. Take for instance the following passage from Letters from the Earth:
“[The Bible] is full of interest. It has noble poetry in it; and some clever fables; and some blood-drenched history; and some good morals; and a wealth of obscenity; and upwards of a thousand lies.” 
Dr. Schwartz holds this same opinion, but holds the further idea that Mark Twain drew continual inspiration from the works of Robert Ingersoll.
From Innocents Abroad, in which Christianity is indicted for having impeded the progress of civilization, to The Mysterious Stranger, in which the concepts of special providence, immortality, heaven, hell, and supernatural beings are reduced to 'empty dreams', Clements relentlessly wove Ingersoll-inspired arguments against Christianity into the fabric of his literary work. His emphasis on the goodness of the present world, his anticlericalism, criticism of the Scriptures, and contempt for the concepts of hell and the atonement, his admiration of Thomas Paine, his belief in human progress and antipathy toward the social conservatism of organized religion, his insistence on religious toleration and separation of church and state, -- all these tendencies reflect his commitment to what he called Ingersoll's "'gospel of free thought' 
Schwartz promotes the idea that Ingersoll’s ideas influenced Clemens heavily in two of his better-known works, The Prince and the Pauper and A Connecticut Yankee in King Arthur’s Court. He does this by citing passages from both men’s works -- and then drawing connections between the two of them. More of the more obvious examples is quoted below:
There is in The Prince and the Pauper a specific instance of borrowing from "Ghosts". Clemens apparently read Ingersoll's lecture in January of 1880. He incorporated into the novel an incident in British history which Ingersoll had recorded. In 1716, according to Ingersoll, a woman and her nine-year-old daughter were hanged for 'selling their souls to the Devil' and for 'raising a storm by pulling off their stockings'. During Tom Canty's brief reign as King of England, a mother and her nine-year old daughter are brought before him. They have been condemned to be hanged for selling 'themselves to the Devil'. Tom is informed that they exerted the power thus obtained to bring about a storm. They did this by 'pulling off their stockings'. 
Schwartz demonstrates similarities in a number of Ingersoll’s and Clemen’s works, showing that Clemens was very much influenced by Ingersoll and that he espoused most of Ingersoll’s ideas himself. He then addresses the fact that Clemens never did a proclamation of disbelief as others were wont to do in the “Golden Age of Freethought”, and explores probable reasons why. Schwartz believes that Clemens refrained from doing so out of “an insurmountable fear of public disfavor.” 
I found the article to be quite interesting. Robert G. Ingersoll represents the very best of humanity to me, and I have enjoyed a lifelong habit of reading Clemen’s works. I think Dr. Schwartz’s article (available at JSTOR) will be of interest to anyone who is fascinated by either Ingersoll or Clemens -- as many freethinkers are.
 Samuel Clemens, Letters from the Earth. Letter III.
 Schwartz, Thomas. “Mark Twain and Robert Ingersoll: the Freethought Connection”. American Literature, Vol. 48, No. 2, p. 184.
 Schwartz p. 187
 Schwartz p. 191