The wonder is not how finite man is made into a divine; rather, it is the infinitely divine becoming genuinely human – new-born infant!
In what could be the earliest confessional statement of the Church outside of the New Testament, the Apostles’ Creed affirms of Jesus in its third line, Who was conceived by the Holy Spirit, born of the Virgin Mary. This is a confession that goes back to the two birth narratives of the Gospels – Matthew 1:18 – 23 and Luke 1:26 – 38. All of orthodox Christendom affirms the virgin conception of Jesus. Why is this significant?
Roman Catholics use this as a basis for the exaltation of Mary in their hierarchy of saints. One must not dismiss this lightly. The recognition of Mary is pronounced in the Lukan narrative. The angel called her, from the well-known KJV translation, blessed among women! (Luke 1:28). Mary herself, conscious of the implication of her favor, said: behold, from now on all generations will call me blessed (1:48).
What must be rejected is the excess to which this Mary-exaltation in the Roman Church was carried. Dogmas developed that gave Mary a position contrary to her original status as a humble maid of Galilee. This includes Pope Pius IX’s declaration of Immaculate Conception as church dogma in 1854. This certainly is against Mary’s confession of God as my Saviour in the Magnificat (Luke 1:47) – owning her need of salvation as herself a sinner. She acknowledges herself as beneficiary of God’s mercy (1:50). One should also deny the tradition of perpetual virginity – to which even some reformers subscribed. It is expressive more of the medieval disdain for sexual union than a serious theological deduction. The time-reference of Matthew should be significant: Joseph took his wife, but knew her not until she had given birth to a son (Matthew 1:24, 25). Mary’s role as a dutiful wife would have normalized after the birth of Jesus.
So why was Jesus conceived of a virgin? Jesus’ was not the only miraculous birth. Even Luke’s narrative of Jesus’ birth was preceded by the account of the conception of Elizabeth leading to the birth of John the Baptist. But all other such miraculous births were of married women who could not be pregnant, or of mothers past their pregnancy age. Such was Sarah’s birth of Isaac. The case of Jesus was unique as the only case of conception by one who was a virgin. Was it necessary? For what?
Continuity and Discontinuity
As the Son of God was to become Man, his humanity must be continuous with the humanity that then existed. He cannot be like Adam, created from the dust, without human parentage. The becoming-Man of the Son of God was to be an act of sharing with flesh and blood (Heb 2:14). Thus, the conceiving by Mary gave him his human substance. The begetting was by the Holy Spirit, but all the conceiving was by Mary. Everything in the process of conception followed the natural human development. This is a marvel in itself. God became everything that humanity undergoes from embryonic to fetal development in the womb! He was, in every way of his human nature, born of a woman (Gal 4:4).
Ancient art has attempted a variety of ways to portray Jesus as super-human: the child with a halo on the manger! Even Martin Luther’s carol says, the little Lord Jesus no crying he makes! Why not? The wonder is not how finite man is made into a divine; rather, it is the infinitely divine becoming genuinely human – new-born infant!
While in every way human, this God-made-man is virgin-conceived, and thus, without a human father. As theologian, GC Berkouwer, puts it:
The human procreation of a human life is not the way of incarnation. At the end of such a way we shall not find Jesus Christ. In analogy with what Jesus says concerning Abraham, we might summarize the relationship with: before Joseph was, Christ is. This is no biological explanation nor does it eliminate the fatherhood, but it recognizes the uniqueness of this birth, which may also be described as a coming into the world. 
Lutheran theologian, Robert Duncan Culver, adds his own take:
The virgin birth provides a reasonable explanation for how a divine Being who is without beginning might take to himself a human nature without the procreation of a new person. 
In being born, Jesus is like any human being. In being born of a virgin, Jesus is not like any human being. He is continuous with humanity, but at the same time, is the Inaugurator of a new humanity.
Humanity without Corruption
The virgin conception of Jesus spares him of that corporate connection with Adam that grounds the imputation of sin. This seems to be the point of contrast in 1 Corinthians 15:47, The first man was from the earth, a man of dust; the second man is from heaven. Both the first Adam and Jesus, as second Adam, are possessor of the divine Image of their respective humanity. In contrast with the first Adam’s humanity as earthly, Jesus’ is heavenly. The latter’s divine image is not just inherited from Adam, but all his own as a man from heaven. While it has nothing explicit to say of the virgin birth, it does corroborate the idea of a different origin of Jesus’ humanity. Says Gordon Fee,
Paul urges that since believers have borne the image of the man of earth, they should also now (because they will) bear the image of the man of heaven. The christological significance of this text is its certain emphasis in context on Christ’s humanity and thus on his being the second Adam, the one who has most truly borne the divine image in his human life. 
This significance of the virgin birth is underscored by Reformed theologian, John Murray,
The Son of God was sent in that very nature which in every other instance is sinful. The Son came by a mode that was supernatural, by a mode consonant with his supernatural person, and by a mode that guaranteed his sinlessness. But he came in a way that preserved fully his organic and genetic connection with us men who are all sinful flesh. He was made of the seed of David, of a seed that was sinful, and of a woman who was herself sinful and afflicted with the depravity incident to fallen humanity. He came into the closest relation to sinful humanity that it was possible for him to come without thereby becoming himself sinful. This is the incarnation that actually occurred. 
William GT Shedd affirms,
The doctrine of the sinlessness of Christ is, thus, necessarily connected with the doctrine of the miraculous conception by the Holy Spirit. The one stands or falls with the other. 
Test of Supernatural Presupposition
If for nothing else, belief in the virgin conception of Jesus tests the supernatural commitment of any theologian. J Gresham Machen spent his life and ministry contending against the Liberals of his day. He saw in the issue of the virgin birth a test case.
It is perfectly clear that the New Testament teaches the virgin birth of Christ; about that there can be no manner of doubt. There is no serious question as to the interpretation of the Bible at this point. Everyone admits the Bible represents Jesus as having been conceived by the Holy Ghost and born of the virgin Mary. The only question is whether in making that representation the Bible is true or false. 
To return to Culver,
In a practical way, the virgin birth tests whether a theologian or a theology is approaching Christianity with wholly naturalistic assumptions or is open to the supernatural… This does not make the virgin birth central to the structure of Christian doctrine and the plan of salvation, but it is a useful test. 
Ultimately, the uniqueness of the birth of Jesus is grounded on the uniqueness of his saving mission. It is not the manger that has become the central symbol of the Christian faith – but the Cross. It is those who see the need of a Saviour from sin who will see the necessity of sinlessness as prerequisite to His saving work. It is those who see their need of salvation from sin who want the One born of a virgin. The Saviour of sinners must Himself be a Man – but not like any man.
 GC Berkouwer, The Work of Christ: 122
 Robert Duncan Culver, Systematic Theology: 48
 Gordon Fee, Pauline Christology: An Exegetical-Theological Study: 119
 John Murray, Collected Writings II: 133
 William GT Shedd, Dogmatic Theology: 639
 Gresham Machen, Virgin Birth: 382
 Culver, 481