Someone asked: "Do you believe that Mary was the MOTHER OF GOD?"
I believe that Mary is the mother of Jesus, who is both God and Man, in two distinct natures and one person. Mary thus had, in her womb, the God-man and she is properly called Theotokos (the God-bearer). Mary, however, was only mother to Jesus' humanity, since only his humanity was derived from her. Thus, Mary was not the mother of the divinity of Christ, and consequently although the phrase "Mother of God" could be interpreted in an orthodox way, it is misleading title that requires clarification.
I think Theodoret put it well in his letter to Ireneaus (Letter 16 - obviously, this is not to the famous Irenaeus, but to another bishop of the same name), when he wrote:
What does it matter whether we style the holy Virgin at the same time mother of Man and mother of God, or call her mother and servant of her offspring, with the addition that she is mother of our Lord Jesus Christ as man, but His servant as God, and so at once avoid the term which is the pretext of calumny, and express the same opinion by another phrase?
It is not a Biblical term and it is not a term favored by many of the fathers before the 5th century. Thus, for example, I cannot find any place in Augustine's genuine works where the term is used, nor likewise among Origen's authentic works. It is not a term used (to my knowledge) by any of the fathers of the first three centuries, including the Apostolic fathers.
Although I have noted above that I could not find the term in any of Augustine's genuine works, I did find a single usage of it among a work of Pseudo-Augustine, De Assumptione Beatae Mariae Virginis (probably 9th century) (Augustinini ... Opera Omnia, Volume 6, 1147). It also used in other ancient forgeries, including the Apocalypse of the Virgin, the Protoevangelium of James, the Revelation of Paul, the Gospel of Nicodemus, and Pseudo-Peter-of-Alexandria.
The same person also asked: "Also, do you believe this title is appropriate to be used towards Mary?"
Hopefully the discussion above has largely answered this question. As a description of the fact that Mary bore Jesus and Jesus is divine, it is descriptive. However, it has a tendency to be understood in a very exalted way, a way designed to treat Mary as particularly special or important. It leads toward the worship of Mary, which is one reason it is inadvisable. Another reason it is inadvisable is that it suggests that Mary is the mother of the Godhead (and of both the human and divine natures of Jesus' person), and not simply of Jesus and particularly his human nature.
It is important to remember what Theodoret wisely noted, namely that with respect to Jesus' humanity, Mary was His mother, but with respect to Jesus' divinity, Mary was His handmaid by her own confession. Is the title potentially ok? I think Turretin put it well when he said:
The "Son of the living God" cannot be called the son of Mary according to that in which he is the Son of God. But because he assumed the human nature from her into unity of person, he is rightly and truly called also the son of Mary in this respect. Thus Mary can truly be called theotokos or "mother of God," if the word "God" is taken concretely for the total personality of Christ consisting of the person of the Logos (Logou) and the human nature (in which sense she is called "the mother of the Lord," Lk. 1:43), but not precisely and abstractly in respect of the deity. Thus she is called the mother of God specificatively (i.e., of him who is God), but not reduplicatively (as he is God).Turretin, Institutes of Elenctic Theology, 13:5:18 (p. 310 in the Dennison edition).