You'll see when I explain.
Some years back, either in my late teens or in my early college years, I was reading a personal experience someone had shared in the New Era magazine. (And I've looked--I cannot find it with the website's search engine.) The girl told of how she sat on the Hill Cumorah and read the Book of Mormon. I believe she read Moroni's challenge and promise in Moroni 10:3-5. And as she sat there, she knew without a doubt that the Book of Mormon is true.
As I read her account, I felt the Spirit remind me that I knew it was true as well. That I had felt just as strongly as she had on the Hill. That I've felt that way standing on that very hill. That I've also felt that way alone in my room with my scriptures. And how I've known? Because the Spirit filled me with, well, I don't know how to describe it. A burning peace? A connected feeling in my heart and mind that I could not in any way refute. It has not happened only once in my life. It has happened often. But that day, as I read the girl's experience and could completely relate, I was reassured all over again--and stronger than any other time that I can remember.
The Book of Mormon testifies of Christ. It's sole purpose is to be another witness of Him. If I knew without a doubt that the book was true, then that meant Christ was real. That He was true, too. I believe the Spirit was once again testifying to me of the truth of the Book of Mormon, because it knew that it would inevitable testify to me of my Savior. Of His love, His power, and His Plan. Because all of that is outlined in the Book of Mormon. And if I know that book is true, then I know of my Savior.