1. To tell them his son Mosiah will be their next king, and
2. To "give this people a name."
They gather in families, in tents around the temple to hear one of the most amazing speeches. It is filled with doctrine like how indebted we are to God and that we should serve Him and our fellowman, that Christ would come (this was before Christ's birth) and "there shall be no other name given nor any other way nor means whereby salvation can come unto the children of men, only in and through the name of Christ, the Lord Omnipotent."
King Benjamin teaches them many things, the result being:
"And we are willing to enter into a covenant with our God to do his will, and to be obedient to his commandments in all things that he shall command us, all the remainder of our days..."(Mosiah 5:5)
For this covenant, King Benjamin gives them a new name: "Children of Christ" and councils them: