And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. (John 1:14 ESV)
First Witness
It might seem like the first witnesses were those that this gospel refers to, the ones who SAW His glory. But before anyone on earth saw anything, God knew. The Father sent the Son into the incarnation.
God the Father (and therefore the threefold Trinity) first witnessed the incarnation. God the Father (and therefore the united Trinity) was first ware of the incarnation. God the Father (and therefore God Himself first knew the incarnation.
And God’s eyes make the best witness. No other witnesses are necessary, really. God saw it. God said it in His Word. God made it be.
And that is enough.
But in His loving grace, God gives us more witnesses. Not to prove anything. But because He wanted THIS creation, THIS miracle, THIS second step to Genesis 1 to be seen by witnesses.