The witnesses to Jesus are: John the Baptist, Jesus’ works, the Father, and the Scriptures.
First Witness: John the Baptist
John the Baptist was introduced in the Prologue as a witness to the light (Jn 1: 6-8). His testimony was presented in John 1: 19-34 and 3: 22-30. Now John the Baptist is called a “burning and shining lamp” (Jn 5: 35). He was not the light but only a lamp (Cf. Sir 48: 1). His function was transitional and temporary (Jn 5: 35). Then, somebody could object to this saying, “John the Baptist’s witness is a human one”. So Jesus moves on to other witnesses.
Second Witness: “Works” of Jesus
Jesus’ “works” which his Father empowered him to complete are the second witness. “Works” (in Greek, Erga), is a word to a certain extent parallel to “signs”. It refers to the ministry of Jesus, but in particular to the miracles. Jesus calls his miracles “works”. If the “sign” implies more the identity of Jesus, “works” refer more to his mission as the envoy of God. Jesus qualifies these “works” as realized through the empowerment by the Father. They point to the authenticity of Jesus’ person and mission as they prove that Jesus is sent by the Father.
Third Witness: the Father
The next witness is Father himself. It goes along with the “works”. John 5: 37, does not seem to be linked to the Baptism scene because unlike in the Synoptic tradition, the evangelist of the Gospel of John does not report a voice from heaven during the Baptist of Jesus (Jn 1: 29-34). The words have the background of Exodus 19, the theophany at Mount Sinai. The testimony of the Father, meant by the evangelist is probably not external but internal, in the interiority of the believer in Jesus. The First Letter of John interprets this as: “…this is the testimony of God that he has borne witness to his Son. He who believes in the Son of God has the testimony in himself.” (1 Jn 5: 9-10; Cf. 1 Jn 2: 14).
Fourth Witness: Sacred Scripture
The final witness is the Scripture. The Jews believed that the Scriptures are the source of eternal life and they come from God. Jesus claims that they speak about him as they contain the messianic prophesies. The prophecies are being fulfilled in Jesus.
Jesus concludes with a critique of the disbelief of Jews. Jesus challenges them. The Jews believed that Moses would defend them before God because they accepted the Scriptures of which Moses is the author. But Jesus tells them that Moses would rather accuse them because they do not believe in him. Their disbelief in Jesus is equal to disbelief in the Scriptures because Scriptures speak about the Messiah. If they accepted the Scriptures, they cannot but accept Jesus too.