Early Christianity saw a wide proliferation of theories and practices concerning baptism, and now many Christians, including Mormons, commonly understand it as a means to repent and wash away one's sins. But the Book of Mormon prophets taught that baptism is a covenant and a witnessing to God that one has already repented and commits to follow Jesus Christ, and that sins are remitted by the Holy Ghost.
Reynolds, Noel B.
"Understanding Christian Baptism through the Book of Mormon,"
BYU Studies Quarterly: Vol. 51
, Article 2.
Available at: http://scholarsarchive.byu.edu/byusq/vol51/iss2/2