Question: "I am a Mormon. Why should I consider becoming a Christian?"
Answer: If we assume that the Bible is the Word of God (both Joseph Smith and Brigham Young believed this), then an examination of the foundational beliefs of Mormonism and the Latter-day Saints (if those beliefs are reliable) should show them to be consistent with what the Bible teaches. Anyone from any religion—or no religion at all—who asks, “Why should I consider becoming a Christian?" should consider the claims of Christianity. For a Mormon asking this question, considering the difference between the tenets of biblical Christianity and LDS philosophy should be the primary area of inquiry. Therefore, we will look at four areas of discrepancy between them.
1) Mormonism teaches a dependence on extra-biblical sources. The Bible teaches that it is sufficient in instruction for all Christian living (2 Timothy 3:16), and God specifically pronounced a curse on anyone who would claim he had authority to add to that which God had revealed in the Bible. In other words, God pronounced his written revelation complete (Revelation 22:18-19). Therefore, there is no reason for God to write more. A God who writes His Scriptures, says they’re complete, and then later realizes He forgot something either didn't plan for the future or didn't know enough to write everything the first time. Such a god is not the God of the Bible. Yet Mormonism teaches that the Bible is only one of four sources, the other three being the Book of Mormon, Doctrine and Covenants, and The Pearl of Great Price. These three came from a single man who declared them to be God-inspired despite their being contrary to the first and only truly inspired text. God Himself pronounced that His written revelation to mankind was complete when He finished the biblical text. To add additional material to Scripture and call it inspired is to contradict God.
2) Mormonism promotes a lesser god. Mormonism teaches that God has not always been the Supreme Being of the universe (Mormon Doctrine p. 321), but attained that status through righteous living (Teachings of the Prophet Joseph Smith, p. 345). Yet who determines what is righteous? That standard can only come from God Himself. So, the teaching that God became God by meeting a predetermined standard originating from God is a contradiction. Additionally, a god that is not eternal and self-existent is not the God of the Bible. Christianity (and, more importantly, the Bible) teaches that God is eternally self-existent (Deuteronomy 33:27; Psalm 90:2; 1 Timothy 1:17) and He is not created but the Creator Himself (Genesis 1; Psalm 24:1; Isaiah 37:16; Colossians 1:17-18).
3) Mormonism teaches an inflated view of humanity that is completely inconsistent with biblical teaching. Mormonism teaches that any human can also become a god (Teachings of the Prophet Joseph Smith, p. 345-354, Doctrine & Covenants 132:20). Yet the Bible teaches repeatedly in hundreds of verses that we are all inherently sinful (Jeremiah 17:9; Romans 3:10-23; 8:7) and that God alone is God (1 Samuel 2:2; Isaiah 44:6,8; 46:9). Isaiah 43:10 records God’s own words: “Before me no god was formed, nor will there be one after me.” How Mormonism can assert that men will become gods in the face of such overwhelming scriptural evidence is a testimony to the depth of man’s desire to usurp God’s place, a desire born in the heart of Satan (Isaiah 14:14) and passed on by him to Adam and Eve in the Garden (Genesis 3:5). The desire to usurp the throne of God—or to share it—characterizes all who are of their father the devil, including the Antichrist who will act on the same desire in the end times (2 Thessalonians 2:3-4). Throughout history, many false religions have played upon the same desire that festers in the sinful flesh of all men. But God declares there is no God but Him, and we dare not contradict Him.
4) Mormonism teaches that we are capable of earning our salvation, again contrary to Scripture (Articles of Faith, p.92, 2 Nephi 25:23). Although we will certainly live differently as a by-product of our faith, it is not our works that save us, but only the grace of God through the faith He gives to us as a free gift (Ephesians 2:4-10). This is simply because God accepts only His own perfect righteousness, which is why Christ died on the cross to exchange His perfection for our sin (2 Corinthians 5:21). We can only be made holy in God's sight through faith in Him (1 Corinthians 1:2).
Ultimately, faith in a false Christ leads to a false salvation. Any salvation that is "earned" is also a false salvation (Romans 3:20-28). We simply cannot be worthy of salvation on our own merits. If we cannot trust God's Word, then we have no basis for trust at all. If we can trust God's Word, then we must recognize that His Word is self-consistent and reliable. While people can corrupt the Bible by creating their own version of it, God is still the God of the universe and capable of preserving His Word in its true form. If He could not or did not preserve His Word, then He would not be God. So, the ultimate difference between Mormonism and Christianity is that Christianity declares a God who is eternally self-existent, who set a perfect and holy standard that we cannot live up to, and who then, out of His great love for us, paid the price for our sin by sending His Son to die on the cross for us.
If you are ready to place your trust in the all sufficient sacrifice of Jesus Christ, you can speak the following words to God: "Father God, I know that I am a sinner and am worthy of your wrath. I recognize and believe that Jesus is the only Savior. I place my trust in Jesus alone to save me. Father God, please forgive me, cleanse me, and change me. Thank you for your wonderful grace and mercy!