I recently debated a Catholic on Facebook. The question was whether Mary is the "Mother of God," and the inferences which Catholics build on that premise. They begin with a deceptively simple argument: If Jesus is God, and Mary is the mother of Jesus, then Mary is the Mother of God.
"Can there ever be a time, place, or location where Jesus is not God? The Church does not teach that Jesus was created in Mary's womb, but that God's word, through the power of the Holy Spirit, was incarnate in the Virgin's womb, and she bore a Son, Jesus. The eternal word was incarnate in her womb and she became his mother."
That suffers from equivocation. There's a sense in which Jesus was created in the womb. The Son qua Son is eternally preexistent, but the Son qua Incarnate has a before and after. Jesus is fully divine, but he isn't simply divine. The hypostatic union was created in the womb. That relation was created. Likewise, the human nature was created.
There was a time when Jesus was not. There was never a time when the Son was not. That's the problem with your equivocal usage. It becomes a shell game.
Jesus isn't simply the Son. Jesus is the Son Incarnate. The Son is timeless, but Jesus has a temporal side as well as a timeless side. Jesus had a first moment of existence, for Jesus is the result of a union in time between the timeless Son and human nature. The relation is temporal, although divine relatum (the Son) is timeless while the human relatum (rational human soul and body) is temporal. The Incarnation is an event. There was no Jesus before Mary became pregnant, although the Son is eternally preexistent. Jesus had a beginning. The Incarnation is a datable event. The Son had no beginning.
"Christ is fully human and fully divine - there can be no separation."
True, However, the question at issue isn't separating the natures but distinguishing the natures. Refusal to distinguish they natures makes you a monophysite.
"God's humanity" is shorthand. What that means, presumably, is that Mary is the mother of his humanity, which is united to his divinity. So she is indirectly the "mother of God" in that convoluted and qualified sense. "Mother of God" is a half-truth.
"Mother of God" has pagan connotations of mother goddesses, like Juno. Their offspring are gods who originate in their mothers.
You can make the title true by layering on caveats, but it's a half-truth because the title itself doesn't contain those caveats. The title itself can be taken in different directions. It dies the death of a thousand qualifications.
Jesus isn't simply God, and God isn't simply Jesus. So, yes, orthodoxy requires qualified usage.
i) For convenience, I've stipulated that Mary was the mother of Christ's human nature. I've said that because it's useful to focus on one issue at a time. If, however, we wish to be theologically precise, that's an oversimplification, and attributes a larger role to Mary than is actually the a case, even in reference to Christ's humanity.
ii) Christ's human nature has two ingredients: a soul and a body. Even in reference to his body, Mary can't naturally be the sole source, because mothers lack the Y chromosome (or SRY gene) to create a male body in the womb. So that had to be supplied directly by God.
iii) Then there's the source of his soul. Some Catholics are Thomists, but Thomistic anthropology can't naturally accommodate the postmortem survival of the soul–as Catholic philosophers like Peter Geach and Mary Anscombe concede.
iv) If you espouse creationism, then Mary was not the source of Christ' soul. Rather, God created that directly.
v) If you espouse traducianism, then we're in uncharted waters. Normally, the soul would be the joint product of a biological father and biological mother. There's no precedent in traducianism to say if a mother could be the sole source of a child's soul. So that, too, might require God to act directly, or at least supplement Mary's contribution.
The upshot is that even in reference to the humanity if Christ, Mary is only a partial source. Her contribution to the human nature of Christ is limited, and for all we know, may well be severely limited.
i) What does "mother" mean? Minimally, a biological mother is a necessary source of origin for a child's body. Clearly, though, Mary is not, in any straightforward sense, the "mother of God" in that respect.
So the Catholic argument boils down to a relation of association. Mary is the mother of God because Mary is a necessary source of origin for Christ's body, which (along with his soul) is linked to the divine Son.
At best, it's one step removed from God. Mary is the mother of God by association: she is the partial source of his humanity, which makes her the mother of God by association via the hypostatic union. When you bother to unpack the claim, it's rather convoluted. A link of a link.
Mary is linked to the deity of Christ because she's linked to the humanity of Christ, which is linked to the deity of Christ.
ii) That, however, is a pretty loose inference. To take a a few comparisons:
a) Aldous Huxley authored Brave New World. Aldous Huxley is the grandson of Thomas Huxley. So he is biologically linked to his grandfather. Therefore, Thomas Huxley authored Brave New World!
But, of course, the fact that the grandson is linked to the grandfather doesn't mean you can attribute to the grandfather the literary production of his grandson.
b) Suppose an automotive engineer designs the brake system for a Ferrari. And the brake system is linked to the steering system, which is linked to the motor, &c.
Does that make him the designer of the Ferrari? No. That inference would commit the composition fallacy.
"Then you are your own authority, yes?"
Are you your own authority when you rely on your own reason to conclude that Rome is the One True Church? You didn't begin with the authority of Rome, because you had to establish to your own satisfaction that Rome had the authority in question. So at that stage of the process you had to exercise your independent judgment by interpreting and assessing the documentary evidence for yourself.