John 1:8

He was not the Light, but he came to testify about the Light.

John 1:20

And he confessed and did not deny, but confessed, “I am not the Christ.”

John 3:28

You yourselves are my witnesses that I said, ‘I am not the Christ,’ but, ‘I have been sent ahead of Him.’

Acts 19:4

Paul said, “John baptized with the baptism of repentance, telling the people to believe in Him who was coming after him, that is, in Jesus.”

Treasury of Scripture Knowledge

Summary

He was not that Light, but was sent to bear witness of that Light.

General references

Bible References

That light

John 1:20
And he confessed and did not deny, but confessed, “I am not the Christ.”
John 3:28
You yourselves are my witnesses that I said, ‘I am not the Christ,’ but, ‘I have been sent ahead of Him.’
Acts 19:4
Paul said, “John baptized with the baptism of repentance, telling the people to believe in Him who was coming after him, that is, in Jesus.”

General references

Mark 11:30
Was the baptism of John from heaven, or from men? Answer Me.”
Acts 13:24
after John had proclaimed before His coming a baptism of repentance to all the people of Israel.