Q: How did Christ, being the Son of God, become man?
A: Christ, the Son of God, became man, by taking to himself a true body and a reasonable soul, being conceived by the power of the Holy Ghost in the womb of the virgin Mary, and born of her, yet without sin.
Philippians 2:7: but emptied himself, by taking the form of a servant, being born in the likeness of men.
I once read a popular "evangelical" author and pastor explain his faith this way. He said for most Christians, their faith is like a brick wall. Remove any of the doctrinal "bricks" in the wall, and the entire wall collapses. He went on to boast how his faith is like a trampoline. If he were to find out that, say, the virgin birth was myth and not true, his Christian faith would not collapse on him, but rather he would simply bounce right back up.
I found it remarkable that this person would use the virgin birth as an example. Why not pick a doctrine like infant baptism, or election, or an issue of church government? Surely, if we were to find out that our understanding on these doctrines were wrong, our faith could stand as well. But the virgin birth? It highlighted to me that this person did not grasp how essential the virgin birth is to Christianity. If it is not true, then our entire faith should collapse!
The incarnation of Jesus Christ (Christ becoming a true, living, breathing, fleshly man) is absolutely essential to our redemption. Only a true man could make atonement for the sins of man. The Old Testament sacrificial system shows us this. The animals could not satisfy completely for the sins of men. They could offer temporary atonement, but they could never atone in an eternal sense. They could not satisfy the justice of God concerning sinful men. Only a man, a man who was without sin, could satisfy God's justice. But man is not without sin, and therefore could have no hope in making atonement for himself. So God would, if he were to redeem man and uphold the covenant of grace, have to provide a way for a true man to make atonement for the sins of God's elect. And he did. God himself became man, took on our flesh, lived the perfect life we could not live, was obedient even to the point of death on a cross, and then died, spilled his own sinless blood, took upon himself the wrath of the Father for all the sins of God's people, and made a perfect, once-for-all-time atoning sacrifice.
And the virgin birth is the means by which God becomes flesh. It is the vehicle of the incarnation. It shows us that Jesus Christ was a true man. He was, as Karl Barth would say, a "real son of a real mother." He didn't simply resemble man, he was man. And yet, he was conceived by God the Holy Spirit. His birth comes directly from God himself. Because of this, he can be fully man and fully God. He can stand in solidarity with sinners, and yet he can be the sinless sin-bearer.
Take away the virgin birth, and you take away our once-for-all-time perfect sacrifice. You take away our hope. You take away our atonement for sin. You take away the idea of God being both the judge and the justifier. Were the virgin birth simply a myth, were it not true, then yes, the entire "brick wall" of our Christian faith would indeed collapse. There is no bouncing back from this reality!