How does one get there? Is it that Jesus was, we’re told, fully human and fully God? By being mother of Jesus, Mary had to have been mother of the God part, too. Is that the justification?
There isn't a "God part" and a "human part". The terms "mother" and "son" describe relationships between persons, not parts of persons. So Jesus is one divine person, who acquired a human nature in the Incarnation. Mary is the mother of that person, and, because that person is a divine person, that person is God.
The contrary error is the error of Nestorianism, which usually ends up supposing that Jesus is two persons, one human and one divine, who are sort of co-located or associated with each other in a mysterious way. That's not orthodox Christology, Catholic, Protestant, or Eastern Orthodox.