You may not make sense on this whole suggestion list but I guarantee you that the following steps are a sure program for getting honest mormon missionaries to seriously reconsider their faith. Had somebody really done a good job like this I would have saved myself years and tens of thousands of dollars of tithes and not gotten into a marriage where I'd eventually be an unbeliever married to a believer.
(A) Ask them to read the following from their Gospel Principles manual chapter 31 on Honesty verbatim:
Complete honesty is necessary for our salvation. An Apostle of the Lord has said: "Honesty is a principle of salvation in the kingdom of God. … Just as no man or woman can be saved without baptism, so no one can be saved without honesty" (Mark E. Petersen, in Conference Report, Oct. 1971, p. 63; or Ensign, Dec. 1971, p. 72). God is honest and just in all things (see Alma 7:20). We too must be honest in all things to become like him. The brother of Jared testified, "Yea, Lord, I know that thou … art a God of truth, and canst not lie" (Ether 3:12). In contrast, the devil is a liar. In fact, he is the father of lies (see 2 Nephi 9:9). "Those who choose to cheat and lie and deceive and misrepresent become his slaves" (Mark E. Petersen, in Conference Report, Oct. 1971, p. 65; or Ensign, Dec. 1971, p. 73). Lying is intentionally deceiving others. Bearing false witness is one form of lying. The Lord gave this commandment to the children of Israel: "Thou shalt not bear false witness against thy neighbour" (Exodus 20:16). Jesus also taught this when he was on earth (see Matthew 19:18). There are many other forms of lying. When we speak untruths, we are guilty of lying. We can also intentionally deceive others by a gesture or a look, by silence, or by telling only part of the truth. Whenever we lead people in any way to believe something that is not true, we are not being honest. The Lord is not pleased with such dishonesty, and we will have to account for our lies. Satan would have us believe it is all right to lie. He says, "Yea, lie a little; … there is no harm in this" (2 Nephi 28:8). Satan encourages us to justify our lies to ourselves. Honest people will recognize Satan’s temptations and will speak the whole truth, even if it seems to be to their disadvantage. People use many excuses for being dishonest. People lie to protect themselves and to have others think well of them. Some excuse themselves for stealing, thinking they deserve what they took, intend to return it, or need it more than the owner. Some cheat to get better grades in school or because "everyone else does it" or to get even. These excuses and many more are given as reasons for dishonesty. To the Lord, there are no acceptable reasons. President Kimball taught that when we excuse ourselves, we cheat ourselves and the Spirit of God ceases to be with us. We become more and more unrighteous. (See Faith Precedes the Miracle, p. 234.) To become completely honest, we must look carefully at our lives. If there are ways in which we are being even the least bit dishonest, we should repent of them immediately. When we are completely honest, we cannot be corrupted. We are true to every trust, duty, agreement, or covenant, even if it costs us money, friends, or our lives. Then we can face the Lord, ourselves, and others without shame. President Joseph F. Smith counseled, "Let every man’s life be so that his character will bear the closest inspection, and that it may be seen as an open book, so that we will have nothing to shrink from or be ashamed of" (Gospel Doctrine, p. 252). http://library.lds.org/library/lpext.dll/Curriculum/home%20and%20family.htm/gospel%20principles.htm/3-unit00050/honesty%20chapter%2031.htm?fn=document-frame.htm&f=templates&2.0 (Gospel Principles, 1997, Church of Jesus Christ of Latter-day Saints, Chapter 31)
B) Tell the missionaries that your research into the Book of Mormon has uncovered the following issues:
1) Latter-day prophets and scripture have clearly taught that the Book of Mormon is an actual historical record (in other words – its not fictional like the "Chronicles of Narnia" or "Star Wars") even though its primary purpose is supposed to be spiritual in nature.
2) Latter-day prophets and scripture have clearly taught that the Native Americans are predominantly Hebrew in origins and the literal descendants of Abraham, Isaac, Jacob and Father Lehi (an actual man who migrated to the Americas from the land of Jerusalem approximately 2600 years ago).
3) Latter-day prophets and scripture have clearly taught that the Book of Mormon Lamanite/Nephite people were a large group or civilization that covered a large portion of the Americas.
4) The Book of Mormon and Latter-day prophets have clearly taught that when Father Lehi arrived in the Americas that there were no other significant groups of people here except for perhaps the Jaredites who were soon extinct.
5) DNA studies have become very accepted in the scientific community for determining the historical migrations and origins of mankind. In fact scientists at BYU are now successfully using DNA evidence to help carry forward the Church’s mission to do genealogy work and redeem the dead.
6) The DNA evidence strongly suggests significant differences between the DNA markers of known Hebrew groups with those of primarily Mongolian/Siberian groups.
7) DNA studies have been used in very recent times to show that there are specific genetic markers (i.e. minor genetic mutations) that Hebrew people have which are not found in other groups of people and that DNA evidence has been used by groups in the reaches of Africa and Asia to prove that they are indeed of Hebrew origins. In these cases the leaders and experts in Israel have changed their opinions and welcomed these groups to immigrate into Israel as "blood brothers/sisters".
8) The DNA evidence strongly suggests that no significant (or even minor) group of Native Americans have been found who have the unique genetic markers found in the DNA of the known Hebrew groups.
9) The DNA evidence strongly suggests that the Native Americans and Mongolian/Siberian groups are closely related and supports the long-held theories in linguistics, anthropology, archaeology and other sciences that the Native Americans primarily descendants of migrants from the Mongolian/Siberian region of Asia.
10) Even though the evidence is very strong its important to remember that research in DNA and Genetics is still progressing and it may still be too soon to draw firm conclusions.
11) The predominantly accepted theory of LDS scholars and apologists is that the "Book of Mormon" people were only a very small group and were in a very small area geographically and that nobody seriously has ever taught that they were ever a large group or large civilization that covered a large portion of the Americas.
12) Another popular theory of many LDS scholars and apologists is that the group of Lehi who were of Hebrew descent arrived in the Americas approx. 600 BC and merged in with the natives who were of Mongolian descent and that’s how the principal ancestors of the Natives were of Hebrew origins even though the scientific evidence shows that they are Mongolian.
13) A much less popular theory of LDS scholars and apologists is that the scientists are actually wrong and that future research will show that the Natives are indeed descendants of Hebrews. Most of the scholars and apologists in the Church who know about the scientific evidences accept what the scientists are finding and ignore the theories that the Native Americans’ principal ancestors were a large civilization who were of Hebrew origins.
14) The Church claims that its true IF and ONLY IF the Book of Mormon is true.
If the missionaries ask how you compiled this list. Tell them you got it off of the Internet from a member of a Stake Presidency who is currently still in just for the sake of family but has read the Book of Mormon about 23 times. I can get you plenty of evidence to backup the following statements and tell the missionaries that they better.
(C) Remind the missionaries what their own Missionary Guide says:
There are many useful tools available to help missionaries (whether they are Full-Time, Stake, or Members) in their callings. One of the most important tools produced by the Church in recent times is the Missionary Guide, 1988 edition. Most Missions and all of the Missionary Training Centers require their full-time missionaries to study the Missionary Guide principles and skills on a daily basis so its safe to assume that the skills listed here from the Missionary Guide are in line with the thinking of the leaders of the Church. A few conclusions that are important to remember for this document are:
Testify of what you teach (rather than off on a tangent)
Maintain an equal relationship and don’t sound better than they are.
Be simple, clear, direct and use words that investigators will understand.
The Spirit testifies of truth so tell the truth in order to have the Spirit which matters the most.
Show those you teach that you are genuinely concerned and sincerely care.
Restate or summarize in your own words what they say to see if you understood them correctly.
Pause after you ask a question to let others gather their thoughts and express their feelings.
Be careful to not jump to conclusions.
Try to understand the thoughts and feelings of others.
Ask questions that help them share their feelings and beliefs to find out if they understand a principle.
Don’t ask ‘yes’/’no’ questions when finding out if someone understands a principle.
Don’t ask manipulative questions designed to make the investigator give the answer you want.
Ask questions that maintain an equal relationship between you and them.
Ask questions that show you respect what they have to say.
Sincerely try to find out their feelings and concerns.
Do not make them feel that you are interrogating them.
"Find Out" skills are important to help them discuss their feelings and prepare them to feel the Spirit.
Empathy is the ability to put yourself in another’s situation and share his thoughts and feelings.
Empathy is also looking at things from the other’s point of view rather than yours.
In showing empathy, you must mean what you say.
Expressing empathy that you do not really feel offends people and does more harm than good.
You are responsible to help others resolve their concerns.
People may hesitate to share their concerns.
Your attitude will often determine whether or not people will share their concerns.
one of your biggest challenges will be finding out what their concerns are.
Show that you care for others by listening to them and showing that you understand them.
Let them know that the concern has not damaged your relationship.
Missionaries often wrongly feel that they need to resolve concerns by bearing testimony or by sharing experiences, scriptures, and examples.
In many situations, however, the best way to resolve concerns is to simply help others discover their own solutions.
Tell them that you are confident he will make the right decisions.
Be able to explain unfamiliar ideas to others and help them discover what they need to do to resolve their concerns.
One of your most important responsibilities as a missionary is to help others identify and resolve their concerns.
D) Let the missionaries know there are several other issues that you have concerns about and that your concerns need to be resolved.
1) Translation of the Book of Abraham facsimiles
2) Plagiarism from Masonry in Endowment
3) Polygamy in Nauvoo, 1835 D&C 101, Breaking Illinois Polygamy Laws
4) Plagiarism from Shakespeare and Anglican Church in Book of Mormon
5) Kinderhook Plates
6) MMM
There are others but this is a good start. If you do a good job you may save 2 missionaries alot of heartache in life from devoting themselves to a false cause. Good luck and let me know if I can be of additional assistance.