John 1:6-8 ESV
There was a man sent from God, whose name was John.
He came as a witness, to bear witness about the light, that all might believe through him.
He was not the light, but came to bear witness about the light.
God sent the light into the darkness, but the darkness would not be able to comprehend the light without help. God would send a man to bear witness and prepare the people for this new thing He was doing.
Spoken as a command in the beginning, it would be in the fullness of time before the feet of the Son would touch the earth. The light is coming and has always been coming into the world. God's rest and creation's end was revealed in the creation account. It is when His light fills the earth that we fully enter His rest.
We, like John, give witness to that same light. We have seen what He is doing and have personally experienced the light of salvation.