Baptism is a reflection or outward ordinance of what one has covenant to God. A sign of what is in the persons heart. One does not receive a forgiveness of sins after baptism, it is not the act, or ordinance of Baptism that bestows that.
Yea, I say unto you come and fear not, and lay aside every sin, which easily doth beset you, which doth bind you down to destruction, yea, come and go forth, and show unto your God that ye are willing to repent of your sins and enter into a covenant with him to keep his commandments, and witness it unto him this day by going into the waters of baptism. (Alma 7:15)This is what Christ did, it was a witness of what he had done. Here is what Joseph said:
Baptism: Is a holy ordinance preparatory to the reception of the Holy Ghost. It is the channel & Key by which the Holy Ghost will be administered The gift of the Holy Ghost by the laying on of hands cannot be received through the medium of any other principle. than the principle of Righteousness, for if the proposals are not complied with it is of no use but withdraws. (here)This next verse shows us it is by the "water" we keep the commandments.
For by the water ye keep the commandment; by the Spirit ye are justified, and by the blood ye are sanctified; (Moses 6:60)So this merely shows that it really is just the beginning of the path leading back to God. But baptism does not get one on the path. I briefly discussed how one is not yet on the path back to God, or the straight and narrow, entered through the gate, until after they have not only been baptized but received the baptism by fire and the holy ghost. Refer to 2 Nephi 31:17-18. This is not the same as "receiving the gift of the Holy ghost by laying on of hands". They are separate events.