When the people heard Benjamin's discourse and asked him how they could "apply the atoning blood of Christ that we may receive forgiveness of our sins, and our hearts may be purified" (Mosiah 4:2), Benjamin told them they (1) must believe in the goodness of God and in the atonement of Jesus Christ, (2) must repent of their sins and forsake them, and (3) must humble themselves before God and ask in sincerity of heart for forgiveness. (Mosiah 4:6-10.)
Although Benjamin did not specifically mention baptism by water and by the Holy Ghost, it seems reasonable to assume that the people whom Benjamin was addressing had already received these ordinances for the following reasons: (1) the Nephites had authority to baptize (2 Nephi 6:2); (2) they plainly understood that baptism was essential to salvation (2 Nephi 9:23-24); (3) apparently the righteous Nephites had practiced baptism from the beginning of the Book of Mormon (2 Nephi 31:17).