What does the inclusion of the word “amen” imply here?

The term is a confession of faith, indicating one’s conviction that the words just spoken are certain. In the present case, John’s inclusion of the closing word amen conveys his own conviction that God’s grace through Jesus Christ is indeed with all, including himself (though still a prisoner). At the same time, the term’s inclusion is intended to prompt John’s readers in the seven churches to echo the same conviction.