John 1:8

John was not the Light, but came to testify about the Light.

John 1:20

And he confessed [truthfully] and did not deny [that he was only a man], but acknowledged, “I am not the Christ (the Messiah, the Anointed)!”

John 3:28

You yourselves are my witnesses that I stated, ‘I am not the Christ (the Messiah, the Anointed),’ but, ‘I have [only] been sent ahead of Him [as His appointed forerunner and messenger to announce and proclaim His coming].’

Acts 19:4

Paul said, “John performed a baptism of repentance, continually telling the people to believe in Him who was coming after him, that is, [to confidently accept and joyfully believe] in Jesus [the Messiah and Savior].”

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 [truthfully] and did not deny [that he was only a man], but acknowledged, “I am not the Christ (the Messiah, the Anointed)!”
John 3:28
You yourselves are my witnesses that I stated, ‘I am not the Christ (the Messiah, the Anointed),’ but, ‘I have [only] been sent ahead of Him [as His appointed forerunner and messenger to announce and proclaim His coming].’
Acts 19:4
Paul said, “John performed a baptism of repentance, continually telling the people to believe in Him who was coming after him, that is, [to confidently accept and joyfully believe] in Jesus [the Messiah and Savior].”

General references

Mark 11:30
Was the baptism of John [the Baptist] from heaven [that is, ordained by God] or from men? Answer Me.”
Acts 13:24
Before His coming John [the Baptist] had preached a baptism of repentance to all the people of Israel.