In my evangelism activities, I have run into Mormons occasionally. I do not look at Mormons as enemies of mine, per se, but I know that they have had deception over them. Their belief system, if understood by the Mormon properly, goes against the Gospel of Jesus Christ. You will not find this understanding though, if your conversation with a Mormon is brief. It is not until you are in a deeper conversation with them, that you can see the difference in the two belief systems.
I have had encounters with Mormons, where they claim that the Gospel that Joseph Smith had been given, is the same one as presented in the Bible. They say they believe in the same Gospel that Christians believe in. But, if that is so, then why would a Mormon believe that salvation can only be found through the Mormon church?
When talking with a couple of different ones, they assure me that they see their need for a Savior, and really do seem to understand the Gospel. They understand that they have sinned against a holy God, and that Jesus provided the sacrifice that was necessary for our salvation. I guess the problem seems to come after that. If we continue to talk about the Bible and salvation, it becomes apparent at some point that we are responsible for doing works that are supposed to supplement our salvation through the Cross. That is probably where the dividing line is. So, do they truly believe that they have to supplement their salvation by doing good works? I think that is exactly what they believe in. Not only that, they are quick to tell you that 'Faith without works is dead'. So, if that is a true belief of theirs, it does differ from the Gospel that is presented in the pages of the Bible.
For by grace are you saved through faith, it is a gift of God. Not of works, lest any man should boast.
Not by works of righteousness which we have done, but according to His mercy He saved us.
If righteousness comes by the Law, then Christ died in vain.
I should list all the verses that have to do with Christ dying for our sins. If I have time, I will do that.
Here is another thought: If Jesus and Lucifer were brothers, then the Bible must be wrong when it says that 'For God so loved the world that He gave His one and only Son, that whoever believes in Him will not perish but will have eternal life." It would stand to reason then, that if Jesus and Lucifer were brothers, then God would be their Father? If God is Jesus' Father, and Lucifer is Jesus' brother, wouldn't that make the devil one of God's sons too? But wait, the verse also says that God gave His only Son. He only has one Son, not two. So there is a contradiction on both counts.
For those of you looking, there is some good info at CARM on dealing with Mormons. Please give feedback if interested in this subject.