True Christians believe in one God
"Hear, O Israel: The Lord our God is one Lord" Moses
has written these words to Jewish tribes in Deuteronomy 6:4. Also God told Isaiah
to write "Is there a God beside me? Yea, there is no God; I know not
any." (Isaiah 44:8). If we go to the New Testament, Paul wrote "One
Lord, one faith, one baptism." (Ephesians 4:5). Also, James, brother
of the Lord Jesus Christ in his epistle wrote "Thou believest that
there is one God; thou doest well: the devils also believe and tremble."
After these words, who can say that true Christians believe in three gods. I challenge you to show me in which book, chapter or verse, the Bible calls Jesus Christ, God the Son or Mary, mother of God. You can never find the words "Trinity", "God the Son", "God the Holy Ghost" or "Mother of God" in any translations or original versions of the Bible. Even the Catholic Bible does not contain these words. The Bible calls God, the creator of the universe, Father because He has created us and loves us like a father loves his children.
Bible teaches that God is a spirit which means we cannot see Him with our eyes but we can feel His presence.
So if God is one, who was Jesus Christ. We all know that all humans have three parts; body, soul and spirit. Jesus had a human body but His spirit was God's spirit. John wrote in his gospel about this matter saying: "...for God giveth not the spirit by measure unto him." (John 3:34). Apostle Paul also teaches that Jesus Christ was the image of the invisible God (Colossians 1:15). God sent a part of Himself in the body of Jesus Christ to earth to redeem the humans from their sins. God can do anything. God can become an ant but still be God. God can become man but still be God at the same time. Nothing is impossible for God. It could not be impossible for Christ to have a dual nature (God and man). Dual nature is observed in the world of quantum mechanics. Light has dual nature (particle nature and wave nature).
You might ask, why we call Jesus, The Son of God? The sonship of Jesus Christ was a spiritual sonship not a physical one (It is blasphemy to say that God had sexual relationship with Mary). God performed a miracle in Mary's life (who was a virgin) and she became pregnant and bore a child and this child was free from sin. All men inherit their name from their father. Jesus also inherited His name from his father. Why is it that all man are named after their father but Jesus is named after His mother (Jesus son of Mary)?
So; who was Mary? She was a blessed woman whom God used for His own glory. She was not much different than any other Jewish woman. She bore a few children after Jesus and that is why Jesus has brothers and sisters. She was not without sin and she needed salvation herself.