The first question to answer is "What is a Christian?"

A Christian is a follower of Christ.

"What is a follower of Christ?"

Anyone who has a saving relationship with Jesus Christ.

For this reason they tried all the more to kill him; not only was he breaking the Sabbath, but he was even calling God his own Father, making himself equal with God.  John 5:18

“Very truly I tell you, whoever hears my word and believes him who sent me has eternal life and will not be judged but has crossed over from death to life.  John 5:24

"For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise them up at the last day.” John 6:40

According to the Bible, Christians must:

• Believe Christ is equal with God
• Hear Jesus' word and believe to receive eternal life.  A person's works do not save him, only his belief.
• Know Jesus raises to life those who look for eternal life.

Mormons want to simplify the definiton of "believe" by saying they believe Jesus existed, exists and walked the earth at one time.  Is that enough?  Jesus said...

“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven.   Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’  Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’  Matthew 7:21-23

Jesus, using some of his harshest words, says to those who KNOW Him and who WORKED in His name, "I have no idea who you are."  Just because you know Christ doesn't mean he knows you.  You can know a celebrity, hear the words of that celebrity, but that celebrity has no idea who you are when you walk up and say "hi."

While Jesus is God and knows everyone, that doesn't mean there is a relationship.  Just before this passage, Jesus speaks of FALSE PROPHETS and how good trees bear good fruit.  So the ones who are speaking "Lord, Lord" are (in context) those who have been led astray by False Prophets thinking they are doing the right thing.  Joseph Smith has been clearly shown as a false prophet so this passage in Matthew 7 is for Mormons.

Mormons cannot be Christians according to Jesus' definition because:

• They deny the Bible as true and so the words of Jesus are suspect.  They trust more in the Book of Mormon.
• They deny Jesus as being an Eternal God, who always lived forever.  They believe Jesus was born in heaven as the first son of God.  He has a birthday and is not eternal.
• They deny Jesus' death as being the final, necessary and complete sacrifice for our sins and believe following the ordinances of the Mormon church truly save you.

Mormons can call themselves "Mormons" because they follow the criteria of their own faith.  But they cannot be followers of Christ since they do not have a relationship with Christ as Christ determines a relationship should be.

They want to be accepted by the mainstream and so want to co-op the term "Christian" as their own.