The Lord chose and established Israel. He would remain committed to them, although they went whoring after other gods.
Moses held the fullness of the priesthood. He conferred blessings upon others. Although Moses was taken from Israel, the blessings of the priesthood remained. Moses blessed Joshua, and Joshua held the blessings of the priesthood for so long as he lived. But the fullness of the priesthood, that portion which permitted a man to see God face to face, was taken with Moses. (D&C 84: 20-25.)
When Joshua died, both the priesthood that left with Moses, and the blessings from that priesthood were lost. What remained thereafter was a lesser form of priesthood called the Levitical or Aaronic Priesthood. This continued to be ministered from Moses until Jesus Christ.
The prophets, however, were something different. They came through diverse families and from unexpected places. They were not part of the leading Levitical families and not even from that tribe on occasion. Their priesthood was not reckoned by what was then on the earth, but was given to them directly from heaven itself. Joseph Smith taught: "All priesthood is Melchizedek, but there are different portions or degrees of it. That portion which brought Moses to speak with God face to face was taken away; but that which brought the ministry of angels remained. All the prophets had the Melchizedek Priesthood and were ordained by God himself" (TPJS, pp. 180-81).
The men who held the higher form of priesthood, the fullness that made it possible for them to behold God face to face, were "all the prophets from Samuel and those that followed after." Having this form of priesthood they could behold God face to face and live. (D&C 84: 22-23.)
The power to see God face to face is not real if the man does not actually behold God face to face. It is powerless. It is theory. It is a notion and not a reality. This priesthood the revelation speaks about is not a theoretical idea, but an actual, real power which allows the person holding it to behold God and live. Therefore, when Christ states that "all the prophets from Samuel and those that followed after" had "testified of [Christ]" this is more than rhetoric. They became prophets by reason of the Lord having appeared and spoken to them; having testified of Himself to them. Therefore their status as prophets and their witness of Him were coequal. They sprang from the very same thing - the same event. This, then, formed the basis for their service as the Lord's prophets. They knew Him. They could testify of what they knew, heard and saw, rather than what they believed to be true from what others had said. God had made Himself known to them.
Christ was confirming that these prophets had testified of Him because He was the one who had called them. He was the one who qualified them. He was the one whose witness and message they bore to others. The testimony of Jesus is the spirit of prophecy! (Rev. 19: 10.) Here He confirms again that those prophets sent by Him have testified they know Him. They do not testify of themselves, but of Him. They do not point to themselves, but they point to Him. They do not promise salvation through themselves, but invite others to come to Christ and be saved. They will understate rather than overstate their calling and standing before God.