Because the Messiah’s mission is in the future, it lies in the prophets’ domain. It is they who see the future and therefore must testify of it. Even though they are testifying of a future event, however, they are also testifying of “things as they really are” (this verse). This timeless and omnipresent nature of truth is also the prophets’ domain. They tell us of things that are eternally true, both current and future (often called the prophetic present).
Jacob identifies himself as a prophet testifying of this future in the company of other witnesses: the prophets of old. Jacob here invokes the Old Testament law of witnesses, which requires that truth be established by two or more testators. Jacob identifies such witnesses and affirms that they abundantly testify to the Messiah’s mission.
In the noncanonical Epistle of Barnabas, which some early churches believed to be the authentic writings of Paul’s traveling companion, appears this passage: “For the Lord hath both declared unto us, by the prophets those things that are past; and opened to us the beginnings of those that are to come.” Both Barnabas and Jacob recognize the inspiration of the ancient prophets, and the additional information given to the current generation of prophets.