His only credential was his message. He came to announce warnings, was rejected, and ultimately killed. He had no success with the people, and made only one convert.
Abinadi is a hinge character around whom the entire remainder of the Book of Mormon will center. His one convert, Alma, will become the spiritual leader of the Nephites, and that convert will become the leading writer of the Book of Mormon. Then his posterity will be the focus of the remaining history of the Book of Mormon.
Abinadi's prophecies were cited from the time he delivered them to the end of the Book of Mormon. But measured by the events of his life, he failed. His one convert fled persecution and hid in the wilderness.
I think there's a profound lesson in Abinadi's appearance and legacy. If the Book of Mormon was edited by those who "saw our day," and was edited to foreshadow our own history, then we ought to be cautious about discarding a message from someone like Abinadi.
The only meaningful credential is the content of the message. Trappings of office, genealogy, name, status, and standing were all irrelevant to Abinadi.