I do not remember the first time I knew that God was real. It seems to me that knowing Him is something I've come to over a life of fighting to reach out to Him, an infinite series of small miracles and preponderance of evidence. I know He has spoken to me, typically, through thoughts and feelings, sometimes through the actions of others, sometimes in still and mighty words.
Debate over the authenticity of the Book of Mormon has been rampant since before it was even removed from the stone-covered box in the side of a hill. I've read and heard it all, how it "cannot be true" because of ABC, and how it "cannot be explained as a lie" because of XYZ.
Despite many people saying "no one can know," I assert that their declarations are useless, for under their definition, no one knows anything. I know that it is true because after extensive testing, after seeking knowledge and pitting that knowledge against the hard rock of experience, I have heard the "song of redeeming love," and tasted God's power.
Elder James A. Cullimore spoke about "The Importance of a Personal Testimony" in each of our lives. One quote caught my attention more than any other.
"The Twelve Apostles are special witnesses of the Savior. I don’t know how many of them have actually seen a personage. They don’t talk about it. But they don’t have to, to receive their special witness that can come by the Holy Ghost."