President Joseph Smith once defined the meaning of baptism by saying,
"[Baptism] is a sign and a commandment which God has set for man to enter into His kingdom. Those who seek to enter in any other way will seek in vain; for God will not receive them, neither will the angels acknowledge their works as accepted, for they have not obeyed the ordinances, nor attended to the signs which God ordained for the salvation of man, to prepare him for, and give him a title to, a celestial glory (HC 4:554)."
This is a bold statement. The President says that the ordinance is a sign set by God for enterance into the Kingdom of God. Our Savior said to Nicodemus,
"Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother’s womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God (John 3:3-5)."
The sectarian will say that he has been born again and accepted Christ into his heart, but one must do more than just lip-service; one must be baptised. Joseph Smith says that "Being born again comes by the Spirit of God through ordinances (HC 3:392)."
This precisely why President Smith said,
"In the former ages of the world, before the Savior came in the flesh, 'the saints' were baptized in the name of Jesus Christ to come, because there never was any other name whereby men could be saved; and after he came in the flesh and was crucified, then the saints were baptized in the name of Jesus Christ, crucified, risen from the dead and ascended into heaven, that they might be buried in baptism like him, and be raised in glory like him, that as there was but one Lord, one faith, one baptism, and one God and father of us all, even so there was but one door to the mansions of bliss (Times and Seasons, Sept. 1, 1842, p. 9)."
This is ratified by both the Bible and the Book of Mormon. In the Bible, we see John the Baptist baptising even before Jesus has wraught His Atonement. 600 years before Christ, Nephi said this,
"And now, if the Lamb of God, he being holy, should have need to be baptized by water, to fulfil all righteousness, O then, how much more need have we, being unholy, to be baptized, yea, even by water! And now, I would ask of you, my beloved brethren, wherein the Lamb of God did fulfil all righteousness in being baptized by water? Know ye not that he was holy? But notwithstanding he being holy, he showeth unto the children of men that, according to the flesh he humbleth himself before the Father, and witnesseth unto the Father that he would be obedient unto him in keeping his commandments (2 Nephi 31:5-7)."
So here we learn that baptism is not only a sign of enterance into the Kingdom of God leading to salvation, but it is also a covenant with God that we will keep His commandments. I know baptism is essential for salvation, and that it is only valid in The Church of Jesus Christ of Latter-day Saints. In the name of Jesus Christ, amen!