No man has seen God at any time.

1 John 4:12 No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us.

The Old Testament authors knew absolutely that God is one. That is the foundation of their faith. You have to better understand what the Word is to make sense of it all.

You are right though, Jesus didn't just show up..God chose him before all time to be our Savior. He is the long awaited Messiah.