What was the problem with the bishops Diodorus of Tarsus, Theodore of Mopsuestia, and Nestorius

  In the name of the Father, and of the Son, and of the Holy Spirit, one God. Amen.

What was the problem with the bishops Diodorus of Tarsus, Theodore of Mopsuestia, and Nestorius (between 300 AD and 431 AD)?

The essence of their heresy (although some of them, like Theodore of Mopsuestia, and Nestorius himself, acknowledge that Jesus Christ has two natures in one person) was that they treated the two natures of Jesus Christ, divinity and humanity, as two persons in one person. This is the core of the heresy they fell into, and it is called the heresy of dualism, which is to treat the two natures of the Lord Jesus Christ as two persons, each separate from the other. That is, they isolated the divinity from the humanity in Christological interpretation and explanation, instead of treating them as two natures in one person. What does it mean to treat them as two natures in one person? It means that since the Incarnation, we should treat the Lord Jesus Christ as one person, without confusion or mixture. This means the inseparable union of the two natures, divinity and humanity. And humanity in the person of Christ, in describing his entire life on earth and in his eternal glory...

For example... Nestorius says in one of the fragments preserved by St. Cyril of Alexandria, in summary:

"In... the Virgin gave birth to a man in whom the Word (God the Word) passed through her and by her... (Note: God the Word was not born of her, but merely passed through her). He continues, saying that she is not the Mother of God... since the divinity of the Word did not take its origin, source, and existence from Mary, but rather from God, because He is God by nature..."

But the correct thing to say is:

God the Word, who has lived from eternity in His divine glory... to whom all the inhabited world listens and submits... for the sake of the redemption of humanity, has submitted from eternity to the divine plan of salvation prepared for Him by the Father for the redemption of humanity... Therefore, the Logos, God the Word, to whom heaven and earth submit, accepted to submit, according to the will of the Heavenly Father, to two of His righteous creatures, namely, the Holy Virgin Mary and Joseph the Carpenter... and that He is raised to obey them with utmost love and humility...

The Incarnation and the union of divinity and humanity in the womb of the Virgin Mary resulted in the formation of the person of the Lord Jesus Christ, which is equal to (the Person of the Lord Jesus Christ = humanity + divinity (without confusion or mixture)). Therefore, when Mary gave birth, she gave birth to the person of Jesus Christ, one Person, with His divinity and humanity as a single whole... Thus, she is not only the mother of humanity but also the mother of divinity. The Lord Jesus Christ is God and man... Therefore, the Virgin Mary is the Mother of God and the Mother of man... But let us note... The Orthodox Churches prefer the title "Mother of God." In all Orthodox Churches, the Virgin Mary is honored with the title "Mother of God" or "Mother of God"...

What does this title mean?

The title does not mean that the Virgin Mary is the mother of the essence of divinity or that she created God from nothing—God forbid, for God is eternal.

Rather, it means that Mary is the mother of Jesus Christ, and Christ, according to the Orthodox faith, is fully God and fully man in one person.

Since the one born of her is the only begotten Son, the incarnate Word, God made man, she is rightly called “Mother of God.”

Is she also the mother of man?

Yes, in terms of human nature, she is the mother of Jesus according to the flesh, meaning that she gave birth to the humanity that was united with the divinity in the person of Christ.

However, the Church prefers to use the title “Mother of God” because it protects the true faith in the unity of the person of Christ—against the Nestorian heresy that separated Jesus the man from Jesus the God.

Glory to God forever.