John the Apostle introduced John the Baptist because John the Baptist bore witness to the light, namely Jesus. John the Baptist was both a model evangelist pointing those in darkness to the light and a model witness providing an excellent example for believers who would follow him. John the Baptist introduced the Light to a dark world. He inaugurated Jesus' ministry. Therefore mention of him was appropriate at the beginning of the Apostle John's account of Jesus' ministry.
1:6 In introducing John the Baptist the writer stressed that God had sent him. He was a prophet in the tradition of the Old Testament prophets who bore witness to the light (Exod. 3:10-15; Isa. 6:8; Jer. 1:4; cf. John 3:17). He was a man in contrast to the Word who was God. The other Gospel writers described John with the words "the Baptist,"but John the Evangelist did not. He probably called him simply John because this is the only John that the Apostle John mentioned by name in his Gospel. He always referred to himself obliquely either as the disciple whom Jesus loved or as the other disciple or in some other veiled way.
1:7 John the Baptist was the first of many witnesses to the light that John the Apostle identified in this Gospel (cf. 4:39; 5:32, 36-37, 39-40; 8:18; 10:25; 12:17; 15:26-27; 18:13-18, 37). The Apostle John frequently used courtroom terminology in his Gospel to stress the truthfulness of the witnesses to the Light. John the Baptist bore witness to the light of God's revelation but also to the person of the Light of the World (8:12). This Gospel stresses the function of John the Baptist as a witness to the light.41The other Gospels also identified his origin and character in their introductions (Matt. 3; Mark 1:1-8; Luke 1:5-24, 57-80).
John the Baptist's ultimate purpose was eliciting belief in Jesus (cf. vv. 35-37). That was also John the Evangelist's purpose in writing this book (20:30-31). Consequently John the Baptist's witness is an important part of the argument of the fourth Gospel. It was not immediately apparent to everyone that Jesus was the Light. John needed to identify Him as such to them.
1:8 Perhaps the writer stressed the fact that John the Baptist was not the Light because some people continued to follow John as his disciples long after he died (cf. 4:1; Mark 6:29; Luke 5:33; Acts 18:25; 19:1-7).
"A Mandaean sect still continues south of Baghdad which, though hostile to Christianity, claims an ancestral link to the Baptist."42
John the Baptist's function was clearly to testify that Jesus was the Light. He was not that Light himself.
The reason the writer referred to John the Baptist in his prologue seems obvious. As the Word came to bring light to humanity, so God sent John the Baptist to illuminate the identity of the Light to people.
