According to Daniel Ludlow, the New Testament mentions the baptism of Jesus Christ, but the exact reasons why the Savior submitted to baptism are not made clear in the Bible except in the Savior's statement that He was being baptized in order "to fulfil all righteousness" (Matthew 3:15).
The Book of Mormon lists several reasons for the baptism of the Christ including the one given by the Savior himself on the eastern continent:
(1) "to fulfil all righteousness" (2 Nephi 31:5); in other words, the Savior was baptized in order to keep the commandments of the Lord.
(2) "He humbleth himself before the Father" (2 Nephi 31:7).
(3) He "witnesseth unto the Father that he would be obedient unto him in keeping his commandments" (2 Nephi 31:7)
(4) "it showeth unto the children of men the straightness of the path, and the narrowness of the gate, by which they should enter, he having set the example before them" (2 Nephi 31:9).
Although Nephi lived over 500 years before the birth of the Savior, yet he knew by the power of revelation that the Savior was going to be baptized. Furthermore, Nephi counseled his people to "do the things which I have told you I have seen that your Lord and your Redeemer should do; for, for this cause have they been shown unto me that ye might know the gate by which ye should enter" (2 Nephi 31:17). This scripture indicates very clearly that baptism was practiced, at least among the Nephites, hundreds of years before the Savior was born.
Other evidence exists, in the scriptures and elsewhere, that baptism was also performed by other groups before the birth of Christ. The Jewish Encyclopedia indicates that baptism was a common practice in ancient Israel: "baptism was practiced in ancient Judaism (Hasidic or Essene), first as a means of penitence . . . to receive the spirit of God, or to be permitted to stand in the presence of God, man must undergo baptism" (Vol 2, page 499). Concerning the mode of baptism, the Encyclopedia says " . . . the [baptism] is only valid when performed by immersion in a natural fountain or stream or in a properly constructed [vessel]. This rule was, of course, also preserved in the temple at Jerusalem" (Vol. 1, pp. 68-69).
The fact that baptism was practiced in ancient Israel might help explain why the Savior was not criticized by the orthodox Jewish people when He was baptized. The Pharisees were very quick to criticize the Savior whenever He did anything which was contrary to their law. However, not a single word of criticism concerning the baptism of Jesus Christ is found in the entire New Testament! [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, pp. 153-155] [See the commentary on 2 Nephi 9:23-24]
According to Cleon Skousen, the Hastings Bible Dictionary describes the Jewish baptism as part of their initiatory ordinance. A male candidate was first circumcised. This was followed in due time by baptism and the offering of a sacrifice. Female candidates were baptized and then they offered a sacrifice. For the baptism, each candidate was taken to "a pool, in which he stood up to his neck in water, while the great commandments of the Law were recited to him. These he promised to keep. Then a benediction was pronounced, and he plunged beneath the water, taking care to be entirely submerged" (see Vol. I, under "Baptism"). [W. Cleon Skousen, Treasures from the Book of Mormon, Vol. 1, pp. 1166-1167]