An example was given of the people repenting after Melchizedek preached to them. Now Alma puts himself in parallel to Melchizedek and cries repentance. If they believe the scriptures, Alma is telling them that they must believe that he has the same authority as Melchizedek to deliver the same message to a similarly wayward people.
Moreover, it isn’t just Melchizedek and Alma; it is angels. An angel appeared to Alma to prepare him to preach this message. An angel appeared to Amulek to prepare him to assist in delivering the message. The message of the coming Messiah is for all to hear, and provides the means for true repentance for those who truly hear that message.