João 1:7

The same came for witness, that he might bear witness of the light, that all might believe through him. He was not the light, but came that he might bear witness of the light.

This parenthesis, including John 1:6, presents the following facts with reference to John the Baptist:

He came from God and was therefore a true prophet.

He was not the light.

His mission was to bear witness to the light.

To bear witness to the light was to bear witness to Jesus Christ.

It is true, of course, that Jesus himself said of John, "He was the lamp that burneth and shineth; and ye were willing to rejoice for a season in his light" (John 5:35); but the apostle John here made a distinction between the light of John the Baptist, which was a dim and borrowed light, and that true light which lighteth every man coming into the world. In no sense could John the Baptist be that light. As the true light, Christ was self-revealed, independent, pre-existent, and eternal. He was the perfect light, in that the source was in himself as identified with the Father.

That all might believe through him ... The purpose of God in sending John the Baptist was that all people might believe in Christ. His was the function of a herald who went ahead of a king to announce his coming and to prepare the popular mind to receive him. John the Baptist effectively discharged that responsibility. The fact that many would not believe was due to hardening and prejudice on their part and not to any fault of the noble herald who went before the Lord in the spirit and power of Elijah.