To testify is to declare a belief about, of, or in something. It is to declare a fact or a truth about something. It is to witness your knowledge of the veracity of something. The word testimony is a noun so someone has a testimony or can declare their testimony by testifying. The word testimony comes from the Latin word testis meaning witness (not to be confused with the male body part!). It is also related to the Latin words for three and stand, implying that the testis, or witness, stands as another (third) witness. We are taught in the Old Testament - testament is the same word as testimony - as well as the New Testament and Doctrine & Covenants that there need to be multiple witnesses to establish the truth of an accusation or declaration; this includes God's word, which shall be established by multiple witnesses (two or three; see Deut. 19:15; Matt. 18:16; D&C 6:28). It is in the bearing testimony of multiple witnesses that truth is established. Even the Godhead, though one in witness and purpose and glory, constitute multiple witnesses. When Jesus Christ was baptized, His Father bore witness to John the Baptist saying, "This is my beloved Son." The Holy Ghost also descended like a dove to bear witness to John of the divinity of Jesus Christ. God's pattern for testifying of His truths is well-established.
When Alma and Amulek started preaching to the people in the land of Ammonihah, the people were astonished that two people bore witness unto them: "And now, when Amulek had spoken these words the people began to be astonished, seeing there was more than one witness who testified of the things whereof they were accused, and also of the things which were to come, according to the spirit of prophecy which was in them" (Alma 10:12). This is one reason LDS missionaries go out two by two - there are multiple people to bear witness, to share their testimony of the truths of the gospel.
In the October 2001 General Conference Elder Richard G. Scott gave this powerful teaching about testimony: "A strong testimony is the sustaining power of a successful life. It is centered in an understanding of the divine attributes of God our Father, Jesus Christ, and the Holy Ghost. It is secured by a willing reliance upon Them. A powerful testimony is grounded in the personal assurance that the Holy Ghost can guide and inspire our daily acts for good. A testimony is fortified by spiritual impressions that confirm the validity of a teaching, of a righteous act, or of a warning of pending danger. Often such guidance is accompanied by powerful emotions that make it difficult to speak and bring tears to the eyes. But a testimony is not emotion. It is the very essence of character woven from threads born of countless correct decisions" (Ensign, Nov. 2001, Scott; emphasis added).
Let me repeat: "[A testimony] is the very essence of character woven from threads born of countless correct decisions." A testimony is based on the goodness of our lives, of our characters, and our actions. Our testimonies are strengthened as we live in accordance to the principles and ordinances of the gospel. In fact, that is the surest way to gain a testimony! Live the gospel principles for which you strive to gain a testimony. If you want to have a testimony of tithing, pay it! If you want to have a testimony of Jesus Christ, live His teachings. If you want to have a testimony of anything else, do it. That is one reason we are taught in Alma 32 to "experiment upon the word." As we test what God has told us, we can know of its truth.
I want to share an experience when my testimony of the prophet Joseph Smith was strengthened. I'll quote from my missionary journal: "Tonight we taught Sarah [name changed] the 1st discussion at the stake center. She bought us pizza! What an amazing discussion. Elders VanBebber, Malie, and I taught it. Sarah is amazing. (Today Elder Malie and I reviewed the 1st [discussion] for companionship study and how we can apply it to Sarah. While doing that I decided to use the Joseph Smith pamphlet and read the Joseph Smith history in the discussion, as we are supposed to [do] but do not always do). [Sarah] used to go to church when she was 8 or so (LDS Church) with some friends. She even wanted to be baptized but told [her mom] and [her mom] said she couldn't go to that family's house any more. What a change [Sarah's mom] has gone through [we taught and baptized her previous to this experience]! I got to teach the Joseph Smith principle. Amazing! While I paused after the First Vision story, the Spirit hit me hard and I even started to cry. That is the first time that has happened to me - getting emotional like that in a discussion. As I testified of Joseph Smith I thought, 'Now I can really, honestly, [and fully] say that I know Joseph Smith is a prophet.' I have always known but now I KNOW! That feeling I received, I shall never forget nor deny."
I have not forgotten that feeling, even 8+ years later. That was one of the singular moments in my life. As I wrote back as a missionary, I have always known Joseph Smith was a prophet. That's not something I have ever doubted. My testimony of his calling as a prophet was based on years of going to church, reading the scriptures, praying, and learning about him. I saw and partook of the fruits the Lord restored through him to the earth. However, before that time I had not had a powerful singular experience like the one I had that evening. That does not mean I did not have a testimony before - I did - but it was strengthened considerably by that experience. What I did not write was how I had been reading my scriptures and praying with a sincere earnestness that I would receive a witness of the gospel. I had not just received that witness out of the blue, although that can happen; testimonies are most often gained through mighty prayer and righteous living. Sometimes - or most times - we must wrestle in prayer as we seek a witness of the truths of the gospel.
For any who want to receive a witness of the truthfulness of the gospel Elder Scott offers His apostolic counsel: "Try reading the Book of Mormon because you want to, not because you have to. Discover for yourself that it is true. As you read each page ask, 'Could any man have written this book or did it come as Joseph Smith testified?' Apply the teachings you learn. They will fortify you against the evil of Satan. Follow Moroni’s counsel. Sincerely ask God the Father, in the name of Jesus Christ, with real intent, if the teachings of the Book of Mormon are true (see Moro. 10:3–5). Ask with a desire to receive a confirmation personally, nothing doubting. There has to be an explanation of that book you can hold in your hand. I know that you can receive a spiritual confirmation that it is true. You will then know that Jesus Christ lives, that Joseph Smith was and is a prophet, and that The Church of Jesus Christ of Latter-day Saints is the Lord’s Church (see introduction to the Book of Mormon, especially the last paragraph). You will confirm that the Savior guides His Church through a living prophet. These truths will become a foundation for your productive life." (Elder Scott, Ensign, November 2003).
I add my testimony to his that all can receive a witness of the truthfulness of the gospel. We can all know that Joseph Smith was a prophet, that the Book of Mormon is God's word. We can know that Jesus is Divine, the Only Begotten Son of God in the flesh, our Savior and Redeemer. We can know that God has a plan for each of us and that plan is to return to live with Him again and to someday see Him as He really is. I know these things and testify of them in the name of Jesus Christ.