Jesus' first witness is John the Immerser. We here see why the Immerser's testimony is so important in John, and is repeated four times (Jn. 1:19-34, 3:25-30, 5:31-36, 10:40-41). John the Immerser was widely accepted as an authentic prophet, even by the critics of Jesus (1:19, 3:33-35; Mt. 21:23-27; Mk. 11:27-33; Lk. 20:1-8). The argument here is that if John, a true prophet, testifies that Jesus is Messiah, then Jesus must be the Messiah, for a true prophet would not make false claims in the name of God.
Jesus' second witness is the Father, a greater witness than John (5:36). How does the Father bear witness that Jesus is the Messiah? There is a possible implication in John of a direct testimony of the Father when John the Immerser saw "the Spirit descended from heaven like a dove, and it remained on [Jesus]" (1:32), but this vision was seen only by John and Jesus (Mt. 3:16; Mk. 1:10; Lk. 3:22). In the Synoptic Gospels, the sign of the dove was accompanied by the voice of the Father, but again this was a personal manifestation, apparently not heard by any other people that were there (Mt. 3:17; Mk. 1:11; Lk. 3:22). Furthermore, the voice of the Father at the baptism of Jesus is not mentioned in John.
In John's Gospel Jesus therefore observes: "the Father who sent me has himself borne witness about me. [Even though] you have never heard his voice, and have never seen his form (eidos)" (5:37). (On the other hand, the disciples will eventually hear the voice of the Father testify of Jesus in John 12:28.) Jesus here appears to be talking strictly to his living audience, since there are numerous examples in the Hebrew Bible of people hearing or seeing God. What Jesus is saying is that no one in this current generation has heard or seen God, as people in earlier generations have.
Here Jesus is talking about the testimony of the Father as manifest in Jesus' "works," meaning his miraculous signs. In first-century Palestine miraculous events were generally viewed as manifestations of supernatural power, either divine or demonic. The reality of Jesus' miraculous deeds or signs was widely accepted, even by his critics. Throughout the Gospels, the critics accuse Jesus of being possessed by a demon, through which he works miracles (Jn. 7:20, 8:48, 52, 10:20; see also Mt. 9:22-27, 12:22-30; Mk. 3:22-27; Lk. 11:14-23). This explanation for Jesus' miracles is also preserved in later Jewish traditions such as the Talmud, and in pagan critiques of Christ such as that by Celsus. Thus, the miracles and signs of Jesus create a dilemma. Either he works his miracles through divine power, or through demonic power. If the former, he is the Messiah, and if the latter, he is "speaking in the name of other gods" (Dt. 18:20) and is thus worthy of death.
Jesus' final witness is the scriptures and Moses. Throughout John, scriptures are invoked as a witness of Jesus (Jn. 1:45, 2:22, 5:45-46, 20:9), though somewhat vaguely. Jesus notes that the Jews search the scriptures because they hope to find eternal life therein (5:39), but the scriptures themselves testify of Jesus, who is the true source of eternal life (5:40). How do the scriptures testify of Jesus? Most Jewish groups of the first century believed that their holy books prophesied of a forthcoming Messiah, although they disagreed as to which scriptures were messianic, and the precise interpretation of these messianic texts. Early Christians read the Hebrew Bible messianically, as can be seen in numerous quotations and allusions to Jesus fulfilling biblical prophecy. Likewise John alludes to Jesus fulfilling several biblical prophecies, which I will discuss later. The point here is that the Hebrew Bible is a witness that Jesus is the Messiah, and if the Jews really believed their scripture, they would accept Jesus as the Messiah. Moses, whom first-century Jews revered as their greatest prophet and lawgiver, also testified of Jesus: "if you believed Moses, you would believe me; for he wrote of me" (Jn. 5:46). Where did Moses prophesy of Jesus? The allusion here is to Deuteronomy 18:15-19.
YHWH your God will raise up for you a prophet like me [Moses] from among your own people; you shall heed such a prophet. . . . Then the YHWH replied to me [Moses]: . . . "I will raise up for them a prophet like you from among their own people; I will put my words in the mouth of the prophet, who shall speak to them everything that I command. Anyone who does not heed the words that the prophet shall speak in my name, I myself will hold accountable."
Jesus is here claiming that he is the "prophet like Moses" into whose mouth YHWH has put his words, and "who shall speak everything that I command" (Dt. 18:18; cf. Jn. 1:21, 6:14, 7:40; Acts 3:22-23). The Jews are therefore required by Moses to hear and obey the words of Jesus. Failure to do so means they "do not believe [Moses'] writings" (5:47). Jesus concludes by noting that Moses himself will accuse the Jews for failure to heed Jesus, just as he did in the wilderness (Dt. 31:19-27). Moses' accusation is: "For I know how rebellious and stubborn you are. Behold, even today while I am yet alive with you, you have been rebellious against YHWH. How much more after my death!" (Dt. 31:27).