The closing admonition is the same as both Mormon and Moroni have previously declared: “Repent all ye ends of the earth, and come unto [Christ], and believe in [his] gospel, and be baptized in [his] name.” That is the gospel. That is the fundamental message of the entire Book of Mormon.
The closing amen finalizes what was originally only the first chapter of the book of Ether, now comprising our Chapters 1–4.