The Arians professed the Jesus was just a man. They used the bible to support such a view and were condemned for heresy. Not because the bible proved them wrong, but because the Church, at Ecumenical councils determined that, from the evidence, Jesus was God.
AND the councils accepted John as an inspired book.
It is not necessary that Luke believe Jesus was God, but that the Church, who is infallible, teach He is God. If you learn the Church has all truth, then this follows. There is no simple path to knowing who He is. Each of us must find him with the help of other Christians.