John 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. John 4:1; Mark 6:29; Luke 5:33; Acts 18:25; Acts 19:1-7).

"A Mandaean sect still continues south of Baghdad which, though hostile to Christianity, claims an ancestral link to the Baptist." [Note: Blum, p. 272.]

Mandaism was a non-Christian type of Gnosticism. [Note: See Morris, p. 57; Beasley-Murray, pp. lvii-lviii.]

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.