I've been asked a few times about witnessing to Mormons. Here's some guidelines from a conversation that happened last night.
1. Don't come across as the heavy right off of the bat. The way I do this is invite them in, act as if I'm glad to see them (it's not an act; I love it when they come) and ask questions about them. Where are they from, how long have they been on their mission (it's a 2 year stint), and, "What's the craziest thing that's happened on your mission so far?" Here in TX they usually have some stories.
2. It helps a lot if you can honestly say you're a student of Mormonism from the historical perspective. I used to live in New York State, not all that far from Palmyra, where Smith claimed to find the golden tablets. I've also been to Susquehanna PA and stood on the spot where Smith claims to have had a visit from John the Baptist. ... See More
As a book dealer, I studied Mormonism from a historical perspective, reading both pro and con material so I could learn who the main people were in case I stumbled across Mormon documents or signed books at auction in Western NY. I have about 12 shelf feet of books on Mormonism, some rare, that I use to read up on it and am always buying more.
You don't need to do all of this to gain credibility, but if you can read enough to have an intelligent conversation with them about Smith, the persecutions the early Mormons faced as they were chased from town to town, you gain respect. Some books I'd recommend on this: Fawn Brodie's "No Man Knows My History" (considered anti-Mormon, but she was a Mormon when she wrote it and then was booted out of the church), Leonard Arrington's "Brigham Young; The American Moses" (pro-Mormon, but fairly balanced), and for a fair treatment of Mountain Meadow's Massacre, Juanita Brooks' work (also pro-Mormon, but Brooks was an excellent historian and was often suspected of being a "Jack-Mormon" but was not).
Since I still sell used\rare books, this is an easy transition because I usually have them sit on the couch in the living room that has floor to ceiling bookshelves with theology and biblical studies titles. They usually ask about them and it opens the door.
Another point that I usually try to make here is that I respect their history, that it is fascinating (it really is, apart from the doctrinal baloney) and that I respect their tenacity and their missionary emphasis (I do...you couldn't get all of the college age kids in one evangelical church to do a two year mission knocking on doors 8 hours a day, much less the thousands they have doing it every day). This is important because when I have to talk about Hell, judgment, the Law, etc., later in the conversation I always preface it with, "I'm not trying to persecute you. I'm simply sharing with you the differences between LDS doctrine and evangelical.
Once I've done all of this, they do their presentation. These days they ask the person they talk to if they would like to pray and I do, asking the Lord to be glorified in our discussion of truth, that He would open eyes to the truth, etc. I let them do their thing, praying that the Lord would protect me from false doctrine as they are sharing. (BTW, I always make sure my kids and wife are in another room. Their doctrine is dangerous and their presentation is very manipulative and emotionally driven.)
When I get the opportunity, I ask them, "What do you think the main difference is between Mormonism and evangelical Christianity?" Here's a list of what I cover:
1. The main one: who is God? Evangelicals say He is eternal God and is Spirit. Their god became a god through obedience, was once a man, and has a physical body. They usually point out that they do not believe in the Trinity. I say that A.W. Tozer said that the most important thing about you is what you believe about God. Then I point out 1 Cor. 6:9-10 and say, "Someone in this room is an idolater. It's either you or me. This is important because whoever it is, they are not going to inherit the Kingdom of God."
2. This brings us to the issue of what is the source of truth. For Mormons, it is the Book of Mormon, Doctrine and Covenants and Pearl of Great Price. It is also their "testimony" that Joseph Smith's vision was legit, the "burning in the bosom" that they experienced when they first heard of the vision or read the BoM.
What I try to deal with here is the idea that: 1) their subjective experience has to be evaluated by an absolute authority: The Word of God. I mention 2 Tim. 3:16-17; 2 Pet. 1:3. 2) The Bible and the LDS doctrine disagree and that is why I go with the Bible. I focus on the doctrine of God here, pointing out Mal. 3:16 about His immutability, Jn. 4:23-24 about His being Spirit (not flesh), etc. This is where I like to use the Commandments to bring knowledge of sin (usually in testimonial form, because now they are trained to deal with Way of the Master people and will gladly admit they are liars, thieves, adulterers at heart...when WoTM was new, they didn't do this). Then I say, "What you all teach is that a lying thieving, adulterous blasphemer can become god. The Bible says He is holy and has always been holy. Big difference."
Then I bring up an example from evangelical Christianity of the foolishness of using subjective experience as a guide for divine revelation. Todd Bentley. Here's a guy who claims he saw angels, had visions and heard from God. He kicked old ladies in the head and punched people in the gut to heal them, saying God told him to do it. I say, "Look, by your standard, I have no basis to say that he's wrong." They will usually say one of two things: 1) you know them by their fruits and 2) you check what they say against the Bible. (Bingo.) The truth is that Bentley did cheat on his wife and was disgraced and that it did NOT add up Scripturally.
I don't go near the moral issues with Joseph Smith here because they are ready for you on that one. I do focus on the Scriptural contradictions. A basic one that really reveals that THEY don't even agree with their own BoM is to ask them, "Where was Jesus born?" They say, "Bethlehem". Correct. But their book claims He was born in Jerusalem (Alma 7:10). Simple stuff, but it's so blatant that it rocks their world. (Josh Williamson taught me this one.)
3. They have a Scripture that says "We're saved by grace after all we can do." This is works righteousness. So I spend some time here on justification by faith alone, esp. 2 Cor. 5:21, but also Is. 64:6 and Rom. 5. This is where I spend most of my time.
4. I also talk about the Deity of Christ here (Jn. 1) and His perfect fulfillment of the Law. Also go to Hebrews 9-10 and point out that He is the fulfillment of the OT sacrificial system, priesthood, tabernacle\temple, etc. If He is the fulfillment, why do you need a temple?
5. After this I try to deal with false converts in Christian churches. They have Eph. 2:8-9 quoted to them all day. They see the Bible belt phonies here in TX and think the sinner's prayer is a joke. I agree with them. Point out Eph. 2:10 and the proper relationship of works. They are the RESULT of justification, not its CAUSE. You have to repeat this because they will agree with you and then turn around and repeat their mantra about doing all we can do.
You have to emphasize that the changed life of a believer is the result of being impacted by eternal God and His righteousness. I like to use Paul Washer's analogy of the logging truck: if I told you I was hit by an 18 wheeler logging truck five minutes ago, you'd expect to see evidence (like a grill imprinted across my forehead). When you encounter the holiness of God, His righteous demands, the person and work of Christ, it is a life changing deal (2 Cor. 5:17). Then I talk about hypocrites and what Jesus said in Matt. 7:21-23.
6. At the end of the conversation I tell them that when I invited them in, I had an agenda. I believe in Hell (Rev. 20:11-15) and I care about them. I owe it to them to tell them the truth if I think they are deceived. At least, if I really believe in Hell and that our gospel is different. I am careful to tell them I have the same concern for many evangelicals (this takes the persecution syndrome out of the equation).
There's more I could write on this, but these are the basics. I hope they're helpful. I find that this approach keeps the focus on the Gospel while dispelling a lot of the stuff they are trained to deal with. At least for the time being.