If one person on the earth has ever received the Son of God in this world (D&C 132: 22-23) in accordance with the will of the Father, (JST John 6: 44) then it must be possible that every person on the earth could do the same thing. God is no respecter of persons. (Acts 10: 34-35) In order to be just, what God does for one person, must be available in the same way for all persons. Joseph Smith said there was nothing he had received from God that was not available to every single person including the least saint as soon as they were prepared to receive it. Joseph Smith received the Son in this world. His calling and election was made sure by revelation and the more sure word of prophecy. (D&C 132: 49-50) If it is indeed "the will of the Father that you receive the Son," (JST John 6: 44) then the will of the Father was in reality accomplished in Joseph Smith's experience. We have a record of it. If the record of Joseph Smith receiving the Son in this world was all we had available to us, it would be enough to show that it is possible for every person, including you, (perhaps especially you,) to receive the Son and His promised redemption in like manner.
The Book of Mormon is called "another" testament of Jesus Christ. We say it contains the "fullness of the gospel" of Jesus Christ. The phrase, "The fullness of the gospel of Jesus Christ," cannot mean everything there is to say or to know about the gospel of the Son of God. This is not the meaning of "the fullness" of the gospel of Jesus Christ. The gospel of Jesus Christ, for every person's individual life, is meant to be a progression of events, ordinances, knowledge, and heavenly gifts, that are designed and intended to result in redemption from the Fall. (3Nephi 27: 13-16) Every person experiences a literal fall by coming to this mortal telestial existence. (Alma 41: 11) The fall separates us from God, and that separation would be permanent but for the atoning sacrifice of The Lord Jesus. The culminating result of all repentance, faith in The Lord Jesus Christ, baptism, the gift of the Holy Ghost, keeping His commandments, receiving all of the ordinances, and all else we do in the name of the Son of God, is to effect the return of the individual to the presence of the Son, who has the authority, and the right, to offer you "redemption." Hence He is called, among His many titles and names, "The Redeemer." This is the purpose and meaning of the gospel, it is all about your "redemption." It is redemption from the Fall by the personal pronouncement of the Son of God to you. This is the express purpose for which we were told in the beginning, "we will provide a Savior for (you)... that (you) may be brought forth by the power of the redemption and the resurrection, and come again into our presence, and with us partake of eternal life and exaltation." This is the fullness of the gospel of Jesus Christ. It is at this point, (when you receive the Son in accordance with the will of the Father,) in your progression that you experience a "fullness" of the gospel. It is called the "fullness of the gospel" because this culmination for you is the full purpose for which The Lord Jesus Christ offered Himself a sacrifice for your sins and transgressions. (2Nephi 2: 6-7) He did that for no other reason than that you might have the opportunity to be redeemed from the Fall and be brought back into His presence. This is the fullness of the gospel of Jesus Christ.
The endowment ceremony in the temple is a depiction of your fall to a telestial world from heavenly beginnings where you enjoyed the presence of God. You are shown through the ceremony, your journey through a lost and fallen world. If you can see it, you are shown your progression through increasing knowledge, and light, and the progressive stages of the gospel of Jesus Christ until the gospel plan brings you to a point where the will of the Father (JST John 6: 44) is ceremonially depicted by you receiving the Son and conversing with Him through a ceremonial veil. All of this sacred ceremony is designed by God to show you what is intended for you in your actual life. It really is the will of the Father that you really do receive the Son, and not just in a ceremonial depiction that is intended to teach you about what needs to become reality in your mortal life. What you learn and repeat ceremonially, should teach you what you need to do in the very reality of your life. Every one of Gods' children is intended to come up to, and to enjoy the fullness of the gospel of Jesus Christ for himself/herself as she converses face to face with The Lord through the veil, not ceremonially, but in reality. Because it is the will of the Father that you receive the Son. (JST John 6: 44)
Be very sure you understand clearly; the ordinances and the ceremony itself do not have power to save you. If you believe you will be saved by the ceremony itself, you may find yourself claiming to the Lord in that day, that this is one of those "wonderful works" done in His name by which you were expecting to be saved in the Kingdom of heaven. (Matt. 7: 21-23) When in fact, if you do not use what you are taught in the ordinances and ceremony to actually "do the will of the Father" in your mortal life and receive the Son, you may then be invited to depart because He never knew you. (Matt. 7: 21)
The Book of Mormon contains the fullness of the gospel of Jesus Christ because it contains a collection of sacred accounts and records of many of those who have done the will of the Father and received the Son in this world. The precious and sacred Book of Mormon does indeed contain the fullness of the gospel of Jesus Christ. Its narrative record and words show us how the fullness of the Gospel, which is the presence of the Son, and His offered redemption, was received by:
Alma the Younger
About 2500 unnamed men, women, and children
The brother of Jared
If all you had was the record left by Joseph Smith that he has received the Son of God and that the Son has pronounced Joseph's redemption from the Fall, (D&C 132: 49-50) then you could be absolutely sure the same is possible for you. Be of good cheer! We have the matchless Book of Mormon which contains too numerous accounts to mention, like those listed above, of those, like Joseph, who have received the Son and His promised redemption from the Fall. The fullness of the gospel of Jesus Christ is repeated throughout the precious record. The Book of Mormon is given to you in the latter day for the express purpose to show you how to do the will of the Father and receive the Son.