This is a statement and promise directly from the Lord. He is saying that if we believe in the Book of Mormon, we will be blessed with the manifestations of his Spirit and we will be given a testimony of the truth of the Book of Mormon. This verse has come true for millions who have studied the Book of Mormon and applied the promise contained in Moroni 10:3-5. The preceding verse speaks of the dramatic moment when the unbelieving at the last day will be shown that the Book of Mormon is the word of God, for ye shall know that it is I that speaketh, at the last day. Hopefully, this realization will come to the reader much sooner than that.
Dallin H. Oaks
"…a manifestation of the Holy Ghost…is given to acquaint sincere seekers with the truth about the Lord and his gospel. For example, the prophet Moroni promises that when we study the Book of Mormon and seek to know whether it is true, sincerely and with real intent, God will ‘manifest’ the truth of it unto us, ‘by the power of the Holy Ghost’ (Moro. 10:4). Moroni also records this promise from the Risen Lord: ’He that believeth these things which I have spoken, him will I visit with the manifestations of my Spirit, and he shall know and bear record. For because of my Spirit he shall know that these things are true’ (Ether 4:11).
“These manifestations are available to everyone. The Book of Mormon declares that the Savior ’manifesteth himself unto all those who believe in him, by the power of the Holy Ghost; yea, unto every nation, kindred, tongue, and people’ (2 Ne. 26:13). ” (Conference Report, Nov. 1996 Ensign, “Always Have His Spirit”)