Previous installments here and here. Guiteau?s trial for the murder of President Garfield began on November 14, 1881, and ran about ten weeks to January 25, 1882.  Direct and indirect references to Mormonism were scattered throughout the trial.
Public sentiment ran pretty emphatically (but not unanimously) against Guiteau. A significant issue early on was seating a reasonably unprejudiced jury (see image below). 
Once the trial was underway, one witness remembered Guiteau ?memorizing some sermons on Mormonism? a few years before the shooting.  Three expert witnesses used Mormonism to illustrate points about the relationships among sanity, delusion, error, and inspiration. 
The bulk of the Mormon references, however, came from Guiteau himself and from the nature of his defense. As a simplification of a 2,600-page transcript, let?s say the defense argued that Guiteau believed God inspired him to shoot Garfield and, therefore, he was not legally responsible for the shooting—that he was, in a narrow legal sense, insane.
Most observers were not impressed with the argument (see image above), but the legal system still had to have it out.  Since the three-year-old Reynolds vs US (1878), or ?Mormon Case,? put limits on how far belief indemnified action, Reynolds played a significant part in the trial.  The Supreme Court had held that the First Amendment protected Mormons? right to believe that polygamy was a duty but that Congress could still restrict Mormons? right to act on that belief. Thus, as Congress had passed a law against polygamy, Mormons could not use ?religious duty? as a defense against prosecution for breaking that law. In the Guiteau case, the prosecution argued that Guiteau was in the same situation as the Mormons: he could believe God wanted him to kill Garfield, but when he acted on that belief he triggered execution of the criminal laws about shooting people.
The following interchange near the end of the trial illustrates both Reynolds?s and Guiteau?s contributions :
Prosecution (Davidge): Now, your honor, I will cite a case—–
Judge (Cox): (Interposing.) It is not worth while to read that case, I have been over it so often.
Prosecution (Davidge): Then I will simply content myself with saying that a Mormon—–
Defendant (Guiteau): (Interrupting.) That reference to the Mormons is very pertinent. The Mormons are breaking the law all the time, yet the Government don?t do anything about it.
Prosecution (Davidge): They break the law all the time, just as the law was broken here, and I am going to tell your honor and these gentlemen what the Supreme Court of the United States said in respect to one of the Mormons. ?[whole page on Reynolds] ? 
Defendant (Guiteau): Mormonism is an institution for the benefit of private lust. My act was a patriotic act for the benefit of the American people. They are well satisfied with the act.
Prosecution (Porter): My associate? [continues as if Guiteau had not spoken]
By the end of the trial the judge did not even need to see the text of Reynolds anymore, the prosecution still had to talk about it for pages at a time, and Guiteau was left to expostulate about Mormonism almost without interference from other trial participants.
Earlier in the trial, Guiteau interrupted his own testimony to say that ?I hope the United States Government will wipe out Mormonism now the Oneida Community has gone.?  Later, the day after President Arthur proposed some actions against polygamy, Guiteau announced—speaking out of turn—that ?I am very glad that General Arthur has rapped those Mormons in his message. I hope he will rap them again. I want him to make it a specialty of his administration to destroy Mormonism.?  Towards the end he interrupted a prosecution speech with, ?Suppose the letter of the law was broken, what of it! Look at those Mormons out there, breaking the law every night. I send greetings to the American Government and General Arthur to attend to those Mormons out there.?  In his own closing remarks Guiteau got sidetracked by the fact that ?thousands of Mormons are daily and nightly violating the law? while the government did nothing. He urged federal officials to ?suppress this gigantic spiritual and social despotism? and then announced, ?If I were President I would clean out these detestable Mormons in some way, and that right speedily. Perhaps in 1884 I shall get a chance at them.? 
He did not get the chance. He was pronounced guilty and sentenced to death on January 25, 1882, as Puck and the Wasp and others had long urged (see image below).  After appeals and processing, he was executed June 30, 1882.
The next post will look at what happened outside the trial and thereafter.
 The formal citation for the trial is United States v. Guiteau, 1 Mackey 498 (D.C. 1882). For my non-legally-trained convenience, all citations will be given as [Volume Number]:[Page Number] of the Report of the Proceedings in the Case of The United States vs. Charles J. Guiteau, Tried in the Supreme Court of the District of Columbia, Holding a Criminal Term, and Beginning November 14, 1881, in 3 volumes, as reported by stenographers Henry H Alexander and Edward D Easton (Washington: Government Printing Office, 1882). The three volumes / parts are Part 1 (p 1-954), Part 2 (p 955-1820), and Part 3 (p 1821-2681). Following actions appear in United States v. Guiteau, 1 Mackey 563 (D.C. 1882), United States v. Guiteau, 1 Mackey 585 (D.C. 1882), and United States v. Guiteau, 10 F. 161 (D.N.Y. 1882). Also: one of the stenographers prepared a condensed account of the trial (800 pages instead of 2,600): HH Alexander, The Life of Guiteau and the Official History of Most Exciting Case on Record: Being the Trial of Guiteau for Assassinating Pres. Garfield (Philadelphia: National Publishing Co, 1882).
 Joseph Keppler, Jr, ?Where Justice Will Have To Look for Jurors Who Have Not Formed an Opinion in the Guiteau Case? Puck 10.242 (1881 Oct 26): front cover. See also Joseph Ferdinand Keppler, ?The Only Road for Him,? Puck 10.249 (1881 Dec 14): centerfold. See editorial comment in No author listed, ?The Only Road for Him,? Ogden Daily Herald, Ogden, UT, 1881 Dec 20 Tue evening, p 2, col 1.
 Fernando Jones, examined by George Scoville, 1881 Nov 28 (1:510); Walter Davidge brought the point up again a few minutes later in cross-examination (1:513). Unless I missed something, all parties seemed to treat the memorization as incidental—as merely a remembered detail that could help establish how well the witness had observed Guiteau. One history of the trial, however, claimed that Jones ?had formed an opinion as to Guiteau?s mental condition, considering him to be of unsound mind, and what some authorities would call in a state of incipient insanity. At the time the prisoner was memorizing lectures on Mormonism and the second coming of Christ, and talked very incoherently.? For them, memorizing lectures seems to have been evidence of early-stage mental illness. Henry G Hayes and Charles J Hayes, A Complete History of the Life and Trial of Charles Julius Guiteau, Assassin of President Garfield (Philadelphia: Hubbard Brothers, 1882), 260. [The same text appears with the same authors, title, date, preface, etc, but different order (p 132).]
 Sane Mormons ?inspired? to enter polygamy: ?There are a number of people who say that they are inspired to do given things; who say, for instance, that they are pleasing God to build churches; that it is God?s will that they should do their duty. These are perfectly sane persons, and you find them every day in your life.? A few lines later, when pressed to give a specific example of a sane person use the word ?inspired,? he brought up Mormon polygamy: ??I have been told that it was a divine inspiration that the man should seal himself or take to himself a certain number of wives. I do not think those people are insane people.? The conversation takes about a page. Allan McLane Hamilton, expert witness, 1881 Dec 21 Wed, 2:1196-1197. Sane Mormons killing as ?God?s service?: After pointing out that ?Saul in persecuting the Christians thought he was doing God?s service; be he was not insane? but admitting that ?Those times were different from the times now,? presents Mormons as a modern example of non-insane religious killing: ?The same thing might be done in these present times, but it is not as common now as it was then. The Mormons are said to have killed people, thinking they were doing God?s service.? Seldon H Talcott, expert witness, under examination by George Scoville, 1881 Dec 23 Fri, 2:1345. Sane Mormons mistaken about truth: ?Persons may be correct or mistaken, and yet there may be no evidence that there is a disease of the brain in consequence. I could illustrate that in the beliefs of Mormons, in the beliefs of Second Adventists, so called, who made all preparations in all sincerity some years ago for what they termed the second coming of Christ, gave away their property, made their children homeless under such a belief. They were mistaken in regard to that, and yet they were not insane at all in a technical sense of the term insanity. There was no disease of the brain existing.? Henry P Stearns, expert witness, 1881 Dec 24 Sat, 2:1368.
 Joseph Ferdinand Keppler, ?It?s No Use,? Puck 10.254 (1882 Jan 18): 309 (front cover). Guiteau, portrayed as an ostrich with ?Murder? written on his tail feathers, puts his head in sand labeled ?Insanity? while death with a lasso approaches on a horse. In other words, the insanity defense won?t work and Guiteau will be executed.
 Reynolds v. United States, 98 U.S. 145 (1878), decided 1879 Jan 06 Mon, cited in Proceedings as ?8 Otto, 145? (before consolidation of Supreme Court reporting to United States Reports, one source was Otto?s United States Supreme Court Reports). Besides shaping the nature of the case, references to Reynolds appeared throughout the proceedings, frequently with explicit reference to Mormons and polygamy. Walter Davidge, prosecution, 1882 Jan 09 Mon, 2: 1791, 1809, 1813; John K Porter, prosecution, 1882 Jan 10 Tue, 3:1822, 1830, 1832; Davidge, 1882 Jan 13 Fri, 3:1918. Reynolds also figured prominently in the instructions to the jury, to which the defense objected as ?irrelevant, and as tending to mislead the jury.? Walter Cox, judge, 1882 Jan 25 Wed, 3:2340; 3:2612-2613.
 Davidge emphasized that ??no matter what his belief was in the inspiration of this Mormon Bible, if he knew the law of the land, and violated the law, he must pay the penalty to that violated law.? Even more important than Mormonism and Reynolds were the McNaughton Rules, which, though not formally binding in the Guiteau case, established the theoretical and linguistic framework for much of the arguments.
 Describing his experience with the Oneida Community: ?It is the greatest system of fanaticism that ever was—that and Mormonism—that ever was concocted by the brain of man. I hope the United States Government will wipe out Mormonism now the Oneida Community has gone.? Charles Guiteau, defendant testifying on his own behalf, 1882 Nov 29 Tue, 1:548.
 Charles Guiteau, defendant speaking out-of-turn, 1881 Dec 07 Wed, 1:852. The ?Annual Message? was what we now call the ?State of the Union? address (though in 1881 it was delivered as a written report rather than a speech). In a message with about 13,320 words, Arthur spent about 440 words on Mormonism, urging more vigorous efforts to suppress polygamy. ?The existing statute for the punishment of this odious crime, so revolting to the moral and religious sense of Christendom, has been persistently and contemptuously violated ever since its enactment. ? I recommend also the passage of an act providing that in the Territories of the United States the fact that a woman has been married to a person charged with bigamy shall not disqualify her as a witness upon his trial for that offense. I further recommend legislation by which any person solemnizing a marriage in any of the Territories shall be required, under stringent penalties for neglect or refusal, to file a certificate of such marriage in the supreme court of the Territory. ? Doubtless Congress may devise other practicable measures for obviating the difficulties which have hitherto attended the efforts to suppress this iniquity. I assure you of my determined purpose to cooperate with you in any lawful and discreet measures which may be proposed to that end.? Chester A Arthur, First Annual Message, 1881 Dec 06.