The Blessed Virgin Mary is the only person whom God has preserved from Original Sin.  She was preserved from Original Sin because she was to be the Mother of the Redeemer.  The Virgin Mary’s preservation from this sin is called her Immaculate Conception.

God, however, did not abandon man in his fall; from the beginning He promised him a Redeemer. (Gen. 3:15).  The Redeemer, Whom God promised to Adam and Eve, came about four thousand years after their fall.  The promised Redeemer, as we know from the Bible and Tradition, is the only Son of God, Jesus our Lord. (John 4:25-26).  The word Jesus means Redeemer or Saviour.  “Thou shalt call His name Jesus; for He shall save His people from their sins.” (Matt. 1:21).  The word Christ means Anointed. 

 In the Old Testament the high-priests, kings, and prophets were anointed with oil.  Jesus Christ is the great High-priest, the King of kings, and the Prophet of prophets.  We know and believe that Jesus is the promised Redeemer, because in Him were fulfilled all that the Prophets foretold about the coming Redeemer. 
The Prophet Micheas foretold that the Redeemer would be born in Bethlehem.

(Mich. 5:2).
The Prophet Isaias foretold that He was to be a great worker of miracles, and a teacher of the people. (Is.35:5-7, Is. 49:1-6). 
The Prophet Zacharias foretold that His disciples were to forsake Him at the time of His Passion (Zach. 13:7). 
The Prophet Ezechiel foretold the foundation and duration of His Church (Ezech. 37:26).

We call Jesus the only Son of God, because He is by nature and from all eternity the only Son of God The Father.  We know that Jesus is the Son of God:
    1) From the testimony of the Heavenly Father:   At the Baptism and Transfiguration of Christ, the voice of the Father declared in the heavens: "This is My beloved Son, in  Whom I am well pleased.” (Matt. 3:17; 17:5).
    2) From the testimony of Jesus: To the words of the High-priest Caiphas, “I adjure Thee by the Living God, that Thou tell us if Thou be Christ the Son of God.”  Jesus answered:  “Thou hast said it.” (Matt. 16:16).
    3) From the testimony to the Apostles:  To the words of Christ, “But whom do you say that I am?“  Peter answered: “Thou are Christ, the Son of the living God.” (Matt. 16:16).
    4) From the testimony of the Church:   The Church has always firmly taught that Jesus Christ is the Son of God. History proves that She has ever defended this doctrine as the chief doctrine of Christianity.