I agree that he may not intend to teach a heresy in Christology. His sermon is mainly a set of "horror quotes" (horror from his perspective) which he takes to be offensively elevating to Blessed Mary, and in his horror he is not unique because many people that I have met, usually people who are not Catholics, have expressed surprise or shock when they've read one of the 19th century or earlier devotional books about Blessed Mary, and I say for myself that I do not entirely enjoy those books because of their flowery adjective laden language. They do not offend me but they are kind of overly effusive in their language. But that is, I believe, a product of the times. Today we pair down adjectives in our prose and effusive praises seem almost like sarcastic mockery.
But to give context to the statement I posted before, John MacArthur's sermon lists a large number of quotes with his own reactions very briefly interspaced among the quotes and then he says
Now if that is not worship, I don’t know what worship is. There is no other definition for that; none whatsoever. Rob Zins writes, “The snowball of Mary in superiority will roll down the slope of Catholic fantasy until she becomes, in their minds, immaculately conceived, sinless, assumed into heaven, and finally redemptress and co-redeemer with Jesus Christ.” And that is exactly right. In fact, Roman Catholics refer to her as Theotokos, God-bearer. They say she gave birth to God and thus is to be elevated and adored. She gave birth to God. That is a terrible misconception. She gave birth to Jesus in his humanity. She did not give birth to God. God was never born.
So, Mr MacArthur rejects "Theotokos" and "God-bearer" just as strongly as he rejects "mother of God" because in his sermon he has constructed a litany of quotes about which he expresses outrage because they offend his idea of how one ought to speak of Mary as a creature, a human being, who he does not desire to elevate beyond what he takes to be the explicit statement of holy scripture.
Nevertheless he is teaching that "
She gave birth to Jesus in his humanity. She did not give birth to God. God was never born." and that is the teaching that the councils of Ephesus and Chalcedon were called to deliberate upon and which they denounced as heresy. Chalcedon's formula states:
Therefore, following the holy Fathers, we all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ, at once complete in Godhead and complete in manhood, truly God and truly man, consisting also of a reasonable soul and body; of one substance with the Father as regards his Godhead, and at the same time of one substance with us as regards his manhood; like us in all respects, apart from sin; as regards his Godhead, begotten of the Father before the ages, but yet as regards his manhood begotten, for us men and for our salvation, of Mary the Virgin, the God-bearer; one and the same Christ, Son, Lord, Only-begotten, recognized in two natures, without confusion, without change, without division, without separation; the distinction of natures being in no way annulled by the union, but rather the characteristics of each nature being preserved and coming together to form one person and subsistence, not as parted or separated into two persons, but one and the same Son and Only-begotten God the Word, Lord Jesus Christ; even as the prophets from earliest times spoke of him, and our Lord Jesus Christ himself taught us, and the creed of the Fathers has handed down to us.6