Moroni did not have an apparatus for quoting scripture as we do. He couldn’t refer to chapter and verse in the stories that his father had written, but he could recall them by the names of those involved and the associated incident. Those who have read the Book of Mormon are familiar with these names and stories. Moroni assumes that his readers would know them, and therefore agree that they had faith prior to the time the miracle was performed.
The interim conclusion echoes the premise: “And neither at any time hath any wrought miracles until after their faith; wherefore they first believed in the Son of God” (verse 18).