What is the basis for saying Mary is not the mother of God from a trinitarian perspective?

Here is what I think. There is a possible misunderstanding between the human language (mother) and the theological language. As an example, here is a true story. There was once the mother of a young Saint woman (Benedetta Bianchi Porro) who reportedly stated at her death: “although I am the mother of my daughter, I must say that I am also her daughter”. The latter meaning is for the spiritual matter. Now for Mary is the same with a plus: Mary is also the Daughter and Spouse of God the Father (here). The matter gets complicated because it has to do with the Incarnation of Jesus and the Trinity as well. Jesus is God, yes he ‘s been existing forever spiritually speaking, but NOT always as flesh . Jesus is the second person of the Trinity, one human person with two natures (human and divine). He has a divine personality in the end, though. Hardly anybody can understand it. Yet, for God this is easy and simple: He is infinite also creatively speaking (the Logos). And so on (and I didn’t mention the Holy Spirit, yet…).

In conclusion, when we say that Mary is the Mother of God this is true materially while spiritually she is the daughter of her Son and so also the Daughter and Spouse of the Father. However, when considering the spiritual side, we must not go back and think materially. Otherwise, we would claim the mother of the young lady who died is a mad person when she says she is the daughter of her daughter. So, with full respect, I think that when people say that Mary is not the mother of God they fall into this mistake.

While "Is Mary the mother of God?" would seem to be a simple question, it's not. This is largely because the meaning of the word "God" is complex and hard to understand. Whether Mary is or is not the mother of God depends very much on which sense you mean "God" in.

All of these are true.

• Mary is the mother of God, because she is the mother of Jesus who is God.
• Mary is not the mother of God, because God the Son existed before she did. You cannot be the mother of someone who existed before you.
• Mary is not the mother of God because she is not the mother of God the Father or God the Spirit, who are equally God.

Historically those who call Mary "Mother of God" have always meant the first sense and are thus not wrong.

You have to be more specific about what you mean by "God" in the question before it can be answered.

Mary as Theotokos (God bearer or mother of God) was a title that was adopted by the Early Nicene Church which brought you the doctrine of the Trinity (as confessed in the Nicene Creed). It was a title first given as a refutation of the Arian heresy which denied the Trinity, as well as the full Divinity of Christ (suggesting he was less than God the Father, was a created being and not eternal).

Historically this practice was contested by Nestorius, Archbishop of Constantinople and this led to a famous controversy in the Church which sparked the Third Ecumenical Counsel. Nestorius first insisted that the term implied Mary gave birth to the Trinity, but after that was refuted then suggested other names for Mary, but in the squabble that ensued it became clear that he was teaching that Christ Divinity was separate from his humanity which was his primary objection to the term Theotokos in the first place.

https://en.wikipedia.org/wiki/Nestorius

I'm very glad you asked this fresh question because correct answers should help clear up particular misconceptions that many Christians suffer from.

In the first instance, it needs to be said that many Trinitarians DO believe Mary to be the 'Mother of God' - taking the phrase at face value and not understanding the theological implications of the title "Theotokos" = 'God-bearer'. Yet again, many other Trinitarians do NOT believe Mary to be the 'Mother of God' in that general sense, even though they do believe she gave birth to the human baby, Jesus, who was God incarnate. They grasp the significance of the original application of that Latin word to apply only to her role "as regards the manhood of the Son of God".

This is what got lost in translation over the years. The Church initially confined "Theotokos" to Mary only with respect to the manhood of the Son of God. They never meant to imply that she was "God-bearer" with respect to the Son's eternal, uncreated deity. The Church, at that juncture, was anxious to protect the deity of Christ. That biblical truth was being attacked by various ones within the Church, and so using the term "God-bearer" with regard to Christ being God in the flesh seemed to be helpful. Alas, the general populace (within Christendom) was not up for the finer points of heavy theology, and just assumed (wrongly) that the Church was teaching Mary to be the mother of God, full stop. Wrong! The Church never, ever taught any such thing, at that time.

Alas, it did not persevere in correcting the error that arose. Perhaps they just felt it had become a lost cause, as worshippers became enraptured with ideas of venerating Mary. Unfortunately, I am away from home right now and cannot source the many quotes I could otherwise provide to explain this. In a few days' time I could add to this answer, if you keep it open till early next week. This is all I can manage, off the top of my head.

Additional details: The Council of Chalcedon, 451, was when Leo, Bishop of Rome, ratified the one personality of Christ and the authenticity and perfection of both his natures - human and divine. This clearly showed that Mary was Theotokos [God-bearer] only as to the manhood of Christ. The populace, however, took this title of Theotokos as an uplifting of Mary's status and venerated her. Clergy, like Nestorius, objected, pointing out that the Bible never says that God was born of the Virgin. It only speaks of the incarnation of the Logos, not of his birth. He said that although such fathers as Origen and Athanasius had used the term theotokos, it had not been incorporated into either the Nicene Creed of 321 or the Constantinopolitan Creed of 381. (Nor is it used either in the Anglican Articles or in the Westminster Confession.)

The term tends to obscure the humanity of Christ and lent itself to the misunderstanding that Mary was the mother of the Godhead. The Chalcedonian definition added the words "as to the manhood" immediately after 'theotokos', which should have put paid to erroneous thinking, but it didn't work out like that among the Catholic populace. A non-kergymatic understanding of theotokos prevents biblical thinking about the amazing role of Mary as the one chosen to provide human nature for the Son of God.

While it is important to maintain that Christ's divinity is not separate from his humanity in the incarnation, and while that unity may be understood to extend back to conception, it is also important to keep in mind what Mary contributed.

It was the power of the Most High which overshadowed Mary and enabled the virgin to conceive. Mary was flesh and flesh was what Mary provided to Jesus. He was the son of David through Mary and the son of God through the Spirit:

Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh; And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead: - Romans 1:3-4

The problem with the title "Mother of God" is that, apart from some very specific contextual knowledge, the offspring of a mother is usually not understood to exist prior to conception and we know that God preexisted Mary. Additionally, motherhood usually also entails a very specific role in the life of a child over a long period of time.

We sparsely see some of this mother/son relationship outlined in the Gospel accounts of the young Christ child. We can never think, however, that Mary provided any motherly benefit to God but only to the incarnated son. Just as it was Jesus' flesh that was tempted such as we are while divinity remained untempted, so it was flesh that was trained and nurtured by Mary: To the eternally begotten Son nothing was added by Mary's ministrations.

There is a danger in elevating Mary, a mere human being, to a position well above her estate and ascribing to her (especially absent some serious context) a role no human could fulfill. There are many other beliefs which have been tacked on to bolster the nomenclature "Mother of God" including (but not limited to) the immaculate conception of Mary herself.

Such things impact the presentation of the Gospel. God was manifest in the flesh and the mystery of godliness is great (1 Timothy 3:16). If Mary was divinely protected from being born with a sin nature in order that she could be the "Mother of God", the "God-bearer", this provides a crack in the necessity of a savior.

For if God could so circumvent the passing on of the "sin nature" to one individual (Mary) born of human parents, allowing the birth of a human (Mary) who did not need salvation, then He could have done the same for any and all of us and Jesus need not have died.