The following are a list of some verses to know when witnessing to Mormons. I will be modifying and updating this list.
Use the King James Version: Since Mormons respect and use the King James Version of the Bible, I recommend that you use this version when witnessing to Mormons. You do not want Mormons to get distracted by the various Bible translations.
Know the Gist: It is not always necessary to have memorized the entire verse word for word. Generally it is okay if you can accurately summarize the verse or give the basic gist of what the passage says. However, I strongly recommend that you at least memorize a few key verses.
Know the Location: It is extremely helpful if you know the exact chapter and verse where a particular Scripture is located. Sometimes it is sufficient to just know the chapter, but it is helpful if you can also know the verse.
One God Only:
Isaiah 43:10: "Ye are my witnesses, saith the LORD, and my servant whom I have chosen: that ye may know and believe me, and understand that I am he: before me there was no God formed, neither shall there be after me."
Isaiah 44:6, 8: "Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God. 8Fear ye not, neither be afraid: have not I told thee from that time, and have declared it? ye are even my witnesses. Is there a God beside me? yea, there is no God; I know not any."
If there were only two Bible verses that you had to know when witnessing to Mormons, I would recommend that you know Isaiah 43:10 and 44:6, 8.
Book of Mormon
Surpisingly to some, the Book of Mormon likewise teaches that there is only one God in all of existence.
Alma 11:26-29: "26And Zeezrom said unto him: Thou sayest there is a true and living God? 27And Amulek said: Yea, there is a true and living God. 28Now Zeezrom said: Is there more than one God? 29And he answered, No."
Eternality of God:
Psalm 90:2: "Before the mountains were brought forth, or ever thou hadst formed the earth and the world, even from everlasting to everlasting, thou art God.
Moroni 8:18: "For I know that God is not a partial God, neither a changeable being; but he is unchangeable from all eternity to all eternity."
God Never Sinned:
Revelation 4:8: "And the four beasts had each of them six wings about him; and they were full of eyes within: and they rest not day and night, saying, Holy, holy, holy, LORD God Almighty, which was, and is, and is to come."
Also compare Moroni 8:18 above which affirms that God is unchangeable. Since God is holy now and is unchangeable, He could not have ever been a sinner.
Jehovah is Elohim:
Deuteronomy 4:35: "Unto thee it was shewed, that thou mightest know that the LORD [Jehovah] he is God [Elohim]; there is none else beside him."
1 Kings 18:39: "And when all the people saw it, they fell on their faces: and they said, The LORD [Jehovah], he is the God [Elohim]; the LORD [Jehovah], he is the God [Elohim]."
Jesus always was God (John 1:1-3, Col. 1:16-17). Since the Bible teaches that Jesus is God (John 10:30, 20:28, Heb. 1:8) and God is eternal (Psalm 90:2), then Jesus must have always been God.
John 1:1-3: "In the beginning was the Word, and the Word was with God, and the Word was God. 2The same was in the beginning with God. 3All things were made by him; and without him was not any thing made that was made."
Colossians 1:16-17: "For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him: 17 And he is before all things, and by him all things consist."
It is important to note that Jesus created all things including Satan. In Mormonism, Jesus is the spirit brother of Satan who was begotten through literal sexual relations between heavenly Father and possibly a heavenly Mother. Therefore, in Mormonism, Jesus did not create Satan. Jesus' spirit body is the result of some sort of union between heavenly Father and one of the heavenly Mothers. Jesus' physical body is the result of some sort of physical union between Heavenly Father and Mary.
Book of Mormon: Moroni 10:32, 2 Nephi 25:23, 1 Nephi 3:7, Alma 11:37, D&C 1:31-33, D&C 58:42-43, D&C 82:7, Alma 34.
Bible: Ephesians 2:8-9, Galatians 2:16-17, John 5:24, Romans 11:6, James 2:14-26.
Hebrews 7:11-12: "If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron? 12 For the priesthood being changed, there is made of necessity a change also of the law."
It is not necessary to actually memorize these passages, but you should at least be familiar with the gist of their teaching, namely that (1) the Levitical priesthood was done away with, and (2) the Levitical priesthood was reserved for Aaron and his descendants from the tribe of Levi, not Gentiles (Numbers 3:5-13, Exodus 28:1-3).
Verses Dealing with Feelings in the Discovery of Truth: Jer. 17:9, Prov. 28:26, Acts 17.
Jeremiah 17:9: "The heart is deceitful above all things, and desperately wicked: who can know it?"
Proverbs 28:26: "He that trusteth in his own heart is a fool: but whoso walketh wisely, he shall be delivered."
Acts 17:11: "These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so." It is important to note that Paul never told the Bereans to determine whether Jesus was the Messiah based on their own personal prayer experience. They were praised since they searched the Scripture daily whether the things Paul taught were true. It is Scripture that is the authority of determining truth, not our own sinful emotional feelings.
1 Thessalonians 5:21: "Prove all things; hold fast that which is good."
1 John 4:1: "Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world."
1 John 5:9-13: "If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. 10He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. 11And this is the record, that God hath given to us eternal life, and this life is in his Son.12He that hath the Son hath life; and he that hath not the Son of God hath not life.13These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God."
The purpose of our existence is to bring glory to God, not to achieve exaltation or human happiness. Revelation 4:11 says, "Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created." Our ultimate fulfillment is actually found when we worship God.