I dislike the word "superior". I believe my church to be the only true and living church upon the Earth. But I do not believe that makes us "superior". We are all sinners and imperfect before God. Thus none of us are "superior" to one another.
But as to the question of why I believe the church I am a member of,
The Church of Jesus Christ of Latter-day Saints, is Christ's true church upon the Earth? There are doctrinal reasons. For example, Paul taught that Christ's church was "built upon the foundation of apostles and prophets, Jesus Christ himself being the chief corner stone." (Ephesians 2:20) The foundation of a building is its most important part. If the foundation is strong the building will stand. If the foundation is compromised though, it is only a matter of time before the building falls. Anciently the corner stone was the part that the rest of the foundation was measured from. If the cornerstone was askew, the foundation would be as well. If the corner stone was straight, so would be the rest of the building. Paul tells us the foundation for the Church of Jesus Christ anciently were His Apostles and Prophets, with Christ Himself being the cornerstone, or chief part.
Nothing in the Bible says that foundation of apostles and prophets and Christ would ever be altered. In fact in indicates the opposite. Paul said this foundation was "For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ", or in other words for the work of the gospel and that it would last "Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ." (Ephesians 4:11-15) So that foundation would endure and lead until all people were united in the one Christian faith, were perfect, and became fully like Christ. Needless to say that time has not come, and therefore that foundation should still exist.
It is part of the message of the Church of Jesus Christ of Latter-day Saints that the Biblical warnings of "a falling away" (2 Thessalonians 2:3) of the ancient Church of Christ, of a "famine of hearing the words of God" (Amos 8:11-12), of the rise of false teachers who taught "damning heresies" (2 Peter 2:1-2, but really the entire chapter) did occur. The original Church of Jesus Christ was lost. The people corrupted the faith, as evidenced by
every epistle Paul ever wrote (which had the express purpose of calling the apostate churches back to the truth), their leaders rejected and persecuted the prophets and apostles (3 John 1:9-11 has an especially good example of this) and lost the fulness of the gospel.
It is also part of our message that this situation did not continue forever. In the spring of 1820 a young boy, Joseph Smith went into a small grove of trees near his house. His intention was to ask God which church he should go to. It was something that had bothered him for a long time. There were so many churches teaching so many things, all claiming to be based solely on the Bible, that it confused him. So after a time of reading the Holy Bible for an answer he came across a verse of scripture, James 1:5, that promised if anyone needs an answer to a question that god will give them that answer through prayer. He said that this verse of scripture entered into his heart "with great force" and "I reflected on it again and again, knowing that if any person needed wisdom from God, I did; for how to act I did not know, and unless I could get more wisdom than I then had, I would never know; for the teachers of religion of the different sects understood the same passages of scripture so differently as to destroy all confidence in settling the question by an appeal to the Bible." So he did the Christian thing and asked God.
The answer he received was undoubtedly one he never expected. Kneeling down, he bowed himself and uttered his first vocal prayer. He called upon God for an answer. In answer he says, "I saw a pillar of light exactly over my head, above the brightness of the sun, which descended gradually until it fell upon me. When the light rested upon me I saw two Personages, whose brightness and glory defy all description, standing above me in the air. One of them spake unto me, calling me by name and said, pointing to the other—This is My Beloved Son. Hear Him!" (Joseph Smith-History 1:16-17) In that grove of trees, God the Father and the Savior Jesus Christ came personally in vision to Joseph Smith. The Father introduced His Son, Jesus, and Christ counseled the young boy. Joseph was told he was to join none of the churches because while there were many good people who were in them, none of them had the fulness of the gospel anymore and had made grievous errors in understand the scriptures, ending up teaching the doctrines of man as opposed the truths of God. Joseph was further told about his calling. God had a great work to do and he was to prepare himself for it. Then the vision ended. But the work was only beginning.
Through Joseph Smith God restored His church. He began by giving forth new scripture, The Book of Mormon. The Book of Mormon is a translation the Prophet Joseph Smith made by the power of God, of ancient records detailing a group of believers here on the American continent before white people came showing that God's saving work wasn't just to a small nation in the Middle East, but a worldwide effort at redemption of the whole human race. It even records a visit of Jesus Christ to these people, after His Resurrection and Ascension in the Old World. Along with this the revelations given to Joseph Smith, and a few of his prophetic successors, were gathered as the Doctrine and Covenants. The Church of Christ was restored as a new Prophet and new Apostles were called to lead it today under the direction of Jesus Christ Himself. Today it is still lead in this same way, with Thomas S. Monson as the current Prophet and leader of the church. God speaks today and gives us guidance for our lives in this modern age. He warns us, teaches us, and saves us. God is not silent nor doth He sleep. He does not leave us to wallow in ignorance or fight over scripture but gives us clear direction and teaching through his modern Prophet and Twelve Apostles.
(To read more of Joseph Smith's story:
http://www.lds.org/scriptures/pgp/js-h/1?lang=eng)
But all this is doctrine. And really even then this is just a small sampling of the doctrine present in the Holy Bible, lost to Christianity, and restored in our day in their fulness as part of the "restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began." (Acts 3:21) This is what I believe. You asked how do I
know its true. That is something related but different.
How do I know its true? In the Book of Mormon the ancient American prophet Moroni (More-own-eye)wrote concerning knowing the truth of something "And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost. And by the power of the Holy Ghost ye may know the truth of all things." This is how I know. When I first came across the church, I listened to its message. I read from the Book of Mormon, studied its words, and pondered about what it (and they) meant and taught. Then truly wanting to know the answer, I asked God. I prayed to know if it was true or not. And He answered my prayer. The Holy Spirit fell upon me, touching my heart and mind. For the first time in my life I communicated with God. I felt the Spirit testify in my heart through feelings and in my mind, through my thoughts, that the Book of Mormon is the word of God and that The Church of Jesus Christ of Latter-day Saints
is Christ's true church upon the Earth today. I not only "feel 'in my heart and in my soul that [my religion] is the true path." but I know in my mind. This became the seed of my testimony, my witness, of the Restoration of the Gospel. Everything I have read and studied and prayed about since then has only confirmed and expanded this original truth.
And what does my church have for you? Well it is my testimony that you to can know all this as well. You do not have to take my word for it. We have missionaries that go out and meet with people and teach them what we believe. But more importantly they can teach you about prayer, how to pray, and how to recognize answers to your prayers so that you on your own can go before God and ask Him yourself if He is real and if this is what He wants for you. You can know of yourself if what I have said is true. You can know the purpose of life, why people exists, what happens after death, the Plan of Salvation, and much more. You can know God knows you, individually and distinctly, and loves you infinitely. You can know relief form the burden of sin and joy in salvation. All this will bring greater joy and knowledge and light and truth into your life more so than anything else, anywhere else, whatever else. This is what is offered you.
If you would like to ask any questions, feel free to message me. I doubt I'd see them on here. Posts get to easily lost on threads like this. That goes for anyone who reads this as well.