The birth of Jesus is an integral part of our salvation. If He were merely human then His death would have been no different than anyone else’s death. But He being born by the power of God through a virgin showed He is indeed the Son of God.
As far back as Eve God had foretold His plan by prophecy of a woman having a child without the involvement of man’s seed.
Gen.3: And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
Looking in Isaiah we find:
Isa.7: Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
The meaning of “Immanuel is God with us” which indeed Jesus is indeed God with us.
Look at Luke chapter One.
Luke 1: And in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary.
 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
 And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
 And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
 Then said Mary unto the angel, How shall this be, seeing I know not a man?
 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
In verse 34 we have the clear definition of “virgin’. Mary indeed was not just a young woman but a virgin.
 Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.
 By this we know that we love the children of God, when we love God, and keep his commandments.
 For this is the love of God, that we keep his commandments: and his commandments are not grievous.
 For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith.
 Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?
Not only do we believe Jesus is the Christ but it is imperative we believe He is the Son of God.
 This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
The Apostle John states the importance of knowing Jesus is born of God and not sinful man.
In a Jewish marriage the husband, upon consummation of the marriage, takes the napkin with the token of his bride’s virginity and displays it. Joseph had this privilege not after his wedding but at the birth of Jesus. At His birth proving Mary was indeed a virgin as He was born of water and blood.
The witness of the scriptures tells us Jesus was conceived by God and born of a virgin. He is the Son of God and not Joseph the carpenter.
 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
 He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.
 And this is the record, that God hath given to us eternal life, and this life is in his Son.
 He that hath the Son hath life; and he that hath not the Son of God hath not life.
 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
The Bible is clear on the matter. Not only do we have the words of the prophecies but we have the testimony of the New Testament account Jesus was indeed born of a virgin. If He were the son of Joseph we would have to look somewhere else for our salvation.
John.3: There was a man of the Pharisees, named Nicodemus, a ruler of the Jews:
 The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him.
 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother’s womb, and be born?
 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
 Marvel not that I said unto thee, Ye must be born again.
Just as Jesus born of the Spirit so are we upon acceptance of Jesus as our Savior born of the Spirit becoming a son of God. (John 1:11)
Jesus was God manifested in the flesh
1 Timothy 3: And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
Jesus not only born of water and blood but the testimony of the water and the blood coming from His side showing evidence He was indeed dead.
John.19: But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water.