The short answer is that the Church experienced many challenges to the idea that Jesus, the son of Mary, was to be regarded as the "Son of God". 

 

     It seems that no one at the time had any qualms about considering Jesus the Son of Mary. So at the very least we can infer that the Jesus of Mary is a historical figure. However, this Jesus of Mary was claimed to be the Son of God who was conceived by the Holy Spirit while Mary was a virgin. 

 

     This claim that the conception of Jesus was by divine action was the basis of the doctrine that Jesus was the Son of God and the Son of Man.

 

     Some writers of the time sought to claim that Jesus was two persons in one body, namely the divine Jesus resided next to the human Jesus within the unity of Jesus' physical body. 

 

     Some others thought that Jesus was never "really" conceived by the Holy Spirit of the Virgin Mary but that this was merely a religious "myth" used to emphasize the unique supernatural life, character and actions of the historical Jesus.

 

     Yet others stated that Jesus was never "uniquely born" of the Holy Spirit but was later adopted into the divinely appointed role played out in His crucifixion and death. 

 

      Still, others claimed that the man Jesus was merely the husk of a divine being who resided within that husk as a "Ghost or phantom or some kind of wholly spiritual being."

 

     Faced with all of these theories, the Christian Church from the beginning regarded Jesus as announced to the Virgin Mary by the Archangel Gabriel to be born by the "power of the Holy Spirit" upon her while she remained a virgin.  Therefore, his conception was virginal, that is, without action of a male. 

 

     Mary herself would have been the only living witness to this event for the Church.  Joseph, Zachariah, Elizabeth and their son John being all dead.

 

     Mary, whose earthly life was much longer than her crucified and resurrected Son Jesus, testified to the factual nature of this event and even filled in the details through the Gospel of Luke who gives his testimony of his interview with Mary as eyewitness of these things. 

 

     Mary would then be either the bearer of Jesus, with an emphasis on his humanity, or she would be the bearer of the divinely conceived God-Man Jesus.

 

     Since Mary's story was regarded as wholly credible, she was considered the "Mother of God" in the special sense that at no time before or after his conception was Jesus not divine, therefore and in answer to those who would detract from this fact of faith, the Church regarded and later declared formally that Mother was indeed the mother of Jesus but more significantly since Jesus was himself God from conception, Mary was rightly considered the Theo Tokas, which is to say the Mother of God, even though she herself was not and is still not considered to be divine.