Why is it that amongst all the Christian denominations in the world, only the Mormons “get” as well as sometimes “lose” their testimonies? How come we never hear about Baptists or Methodists losing their testimony of Christ? Or in the things that they believe? But we often hear about LDS people who sometimes lose their testimonies of Joseph Smith, the LDS church or the Book of Mormon. Why is this so? Why is a testimony of Christ, Christianity and the gospel as taught in the Bible practically impervious to loss (no matter how bad you live), but testimonies of the true church, the Book of Mormon and Joseph Smith are so fragile and very easy to lose through neglect or transgression?
Critical to the answer of your question is the definition of the term, testimony. The dictionary definition will do for our application, which is, “firsthand authentication of a fact.” By firsthand is meant the declaration of personal experience, not the repetition of the word of another. Such a declaration is termed hearsay testimony, and is not judicially valid. So a testimony of the truthfulness of the gospel as declared in the Mormon Church is a firsthand witness of the Holy Spirit that the gospel is true, that the Church of Jesus Christ of Latter-day Saints is indeed the kingdom of God on the earth, that the restoration of the gospel was brought about through the instrumentality of the prophet Joseph Smith, that the holy priesthood restored to the earth through the prophet Joseph Smith is the only valid representation of Deity in the earth, that all the presidents of the Church following the prophet Joseph were and are indeed prophets, seers and revelators, speaking officially to the Saints and to the world in behalf of the Lord, Jesus Christ.
Key to that statement is that this witness, or this testimony, is a communication by and through the Holy Ghost, representing the Father and the Son.
Now the question, would the Holy Ghost ever testify to men that any other Christian denomination in the world is the true church? Of course not! The Spirit cannot lie. So in the first place, the sectarian world has never received from the Holy Spirit any sort of witness that their church is the true church. In fact, that is not a general belief by almost all the Christian sects except the Catholic Church, that does make the claim that they are the true church, and the official representatives of God to both the church and the world. However, the members of the Catholic Church cannot receive a testimony of its truthfulness from the Holy Spirit, simply because it is not God’s organization. It represents only the apostate remnants of the church established by the Savior and the original apostles in the meridian of time. As mentioned in 1 Corinthians, the things of God can only be understood by the Spirit of God—
For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, except he has the Spirit of God (JST 1 Cor 2:11).
So if a person who has a testimony of the truthfulness of the gospel borne by the Spirit becomes unworthy to receive the influence of the Spirit, when the Spirit leaves, that knowledge which is given by the Spirit also leaves, and the person no longer knows what he knew before. Anyone and everyone who loses their testimony does so through the process of violating the principles of righteousness to the extent that they no longer are in possession of the gift of the Holy Ghost that was conferred on them at their baptism.