Mormons often argue that because they live good lives, they pass the test given by Jesus in Matthew 7:15-23 to determine whether or not a person is following a true prophet of God:
A good tree cannot produce bad fruit, nor can a bad tree produce good fruit. . . . So then, you will know them by their fruits.
This single test of behavioral fruit (they limit this passage to actions, to the exclusion of doctrine) proposed by many Mormons is necessary (since it's true that we ought to see a change in behavior over time in those who truly love Jesus and His words), but it's not sufficient. The Bible explicitly gives two doctrinal fruit tests for false prophets:
1. Do they teach the truth about God as revealed in the Bible? Deuteronomy 13:1-3 - "If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, and the sign or wonder comes true, concerning which he spoke to you, saying, 'Let us go after other gods (whom you have not known) and let us serve them,' you shall not listen. . . ."
2. Do they teach the true Gospel as revealed in the Bible? Galatians 1:8 - "If we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed!"
If we were to water down these tests for true followers of Christ to a simple, "Do they behave nicely as compared to many other people in the world?" we'd end up including in this group some very ethical people who deny Jesus flat out (who doesn't know at least one kind, giving person who isn't a Christian?); and Jesus can't have meant for us to do this since "everyone who beholds the Son and believes in Him will have eternal life" (John 6:40), and the necessary fruit--the thing we should do "that we may work the works of God"--is "that [we] believe in Him whom He has sent" (John 6:28-29).
The question of whether or not a person knows and proclaims the true Jesus is the key element in determining the quality of his fruit because no matter how a person compares to others in behavior, nobody compares to the perfect righteousness of God; only those joined to the true Jesus (to whom Jesus will not say "I never knew you") will have Christ's righteousness given to them--the perfect righteousness necessary to enter heaven. For Jesus also said, "Unless your righteousness surpasses that of the scribes and Pharisees, you will not enter the kingdom of heaven" (Matthew 5:20).
If Jesus, the person, is all-important in our salvation, we can't ignore the fruits of true relationship (i.e., correct doctrine--a true knowledge of Jesus Himself). Doctrine is an integral part of the fruit we're evaluating. If it can be demonstrated that LDS teaching is contrary to the Bible in the areas of God and the Gospel, the next step is to explore whether or not the Bible has been altered. If the Bible has not been altered, then Joseph Smith is not a true prophet, and those who follow his Jesus are saying, "Lord, Lord," and do many things in His name but will not enter into heaven because they do not know Him.