One of the differences between Catholics and many other Christian groups is that many others do not want to call Mary "The Mother of God." But such a title is proper to Mary and in fact necessary for us to maintain an orthodox faith in Jesus Christ.
In the early part of the fifth century, the Bishop Nestorius made the statement that "you could not call a three month old baby God." He was adamant that although Mary was the Mother of the Christ (the Anointed One), it was not proper to call the infant Jesus "God." So to Nestorius, Mary was not the Mother of God. Nestorius proposed that Jesus the Man became God. Such a view is opposed to Holy Scripture. John's Gospel clearly states that the Word became flesh and dwelt among us (John 1:14) and not that the flesh became Word sometime after he was born.
The Pope and the other Bishops reacted against Nestorius. He was ordered to recant his position, or be removed as a bishop. In order to clarify the issue, the Council of Ephesus was called in the year 431. At the council Nestorius was able to clarify his position, which was condemned by the bishops. Presently the pope's legates arrived with the letter. Nestorius failed to change his ways and that is his problem. But an upshot of the Council was that Mary received the title of Theotokos or God-Bearer, which is generally translated Mother of God because Mary simply did not carry God around in her arms, but it was from her that God was born as a human being. That Mary should be called the Mother of God fits with Scripture. When Mary visited her kinswoman Elizabeth, the latter under the inspiration of the Holy Spirit said: who am I that the mother of my Lord should come to me? (Luke 1:43). Even if Elizabeth did not know what she was saying (and that really does not matter anyway), the angel who announced to the shepherds about the birth of Jesus told them that a savior has been born for you who is Messiah and Lord (Luke 1:11). There were not two births, or two mothers, or two beings, it was one birth, one mother and one savior who was born. Both the angel and Elizabeth were referring to God when they said Lord.
Thus, calling Mary the Mother of God is essential to our correct understanding of Jesus the Christ. For in Jesus the Word became flesh. That is, Jesus is the Son of God. He is completely divine. There is no lack of God-ness in Jesus. Thus Jesus is completely Holy, Completely Loving, Completely Perfect, Completely Good, All Powerful, All Knowing, and so forth. In every way that God the Father is God, so is Jesus His Son, except the Son is not the Father. In the Creed we say Jesus is God from God, Light from Light, True God from True God, begotten, not made, one in being with the Father...
Yet from the first moment in which the Holy Spirit overshadowed the Virgin Mary and she conceived Jesus in her womb, at that moment the Son of God became a human being. There was never a moment in Mary's womb in which Jesus was not God. Nor was there any moment in which He was not human. Jesus' humanity is complete and total. He is like us in all ways (except for sin). Jesus possessed every need to grow and develop. He possessed every characteristic of a human being and every weakness (except He never sinned) and strength.
The person Jesus whom Mary bore in her womb was and is completely both human and divine. in other words, in Jesus, God has joined His divine nature with our human nature. And such a union has consequences. For in joining our nature to His, Jesus has transformed what it means to be human. That is why He is called the New Adam (see 1 Corinthians 15:45).
In the ancient world in which the early Church existed, there were certain public practices against which the Church fought. One of these was the exposing of children. For example in the city of Alexandria, if a family did not want a child, they would just leave the child outside the gate next to a wall in the evening. During the night, wild animals would take the child away. So in the morning, there was no unwanted child. Furthermore, there was the practice condemned in the Bible as sorcery (see Revelations 21:8). The word translated sorcery is rendered in Greek pharmakea, from which we get pharmacy. This was a description of those who would, among other things, make potions to cause a baby to disappear, that is abortafacient drugs.
But if the Lord God humbled Himself to become a weak and vulnerable human being, even allowing Himself to go through the stages of gestation in the womb, it changes everything. We cannot look at others the same way. There is no such thing now as a throw away human. In Genesis, it states that we are made in the image and likeness of God. But in the Incarnation of Jesus Christ, human beings have been elevated. Paul wrote to the Galatians that God sent His Son, born of a woman, born under the law... so that we might receive adoption as the children of God (see Galatians 4:4-7). It is God's intention to share everything of His divine nature with us, as He has shared everything of our human nature. John said Beloved, we are God's children now; what we shall be has not yet been revealed. We do know that when it is revealed we shall be like him, for we shall see him as he is (1 John 3:2). Sharing in God's nature is the true destiny of every human being. Not to become "gods" controlling planets or some such, but to become one with God as His adopted children.
Another consequence is that we are obligated to share this good news with every other person in the world. God wills that all men be saved and come to the knowledge of the Truth (1 Timothy 2:4) Woe are we if someone does not receive the Gospel because we were silent.
Several years ago someone questioned me about calling Mary the Mother of God and said that they could not do such a thing. So I asked, are you saying that Jesus is not God? My friend said no. So I asked him, are you denying that Jesus is human? He said no, not that either. So I said do you doubt that Mary is His mother? Again he said of course not! THEN WHAT'S THE PROBLEM?