It is in this context of a future event that has current implications that we see where Jacob is going in this verse. Because the mission of Christ is in the future, it is the domain of the prophets. It is they who can see the future who must testify of it. Even though they are testifying of a future event, however, they are also testifying of "things as the really are." This eternal and present nature of truth is also the domain of the prophets, that they may tell us of the things that are eternally true, both current and future.
Jacob places himself in that context as a prophet testifying of this future, but he notes that he is joined by other witnesses - the prophets of old. In so stating, Jacob is invoking the Old Testament law of witnesses, requiring at least two, and preferably more, to testify. His word is that there are such witnesses, and those abundantly in number and faith have testified to the mission of this Messiah.
The essentiality of the witness of the prophets is echoed in the epistle of Barnabas, which some early churches believed to be the authentic writings of Paul's traveling companion. In that epistle, Barnabas writes: "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. (Barnabas 1:8. In: The Lost Books of the Bible and the Forgotten Books of Eden. World Publishing Company, 1973, p. 145). Both Barnabas and Jacob recognize the inspiration of the ancient prophets, and the additional information given to the current generation of prophets.