Heresies never really go away, do they? They just get repackaged, repurposed, and recycled for every generation. And the Church must always stay on guard, always ready to defend the faith once delivered against new affronts by old ideas.
Around Christmas time each year, for instance, we get to see the new variations on the historical understanding of the birth of Christ (just as every Easter we get to re-experience the annual attacks on the historicity of the resurrection). Just last week, I responded to this question by someone on Twitter: "If you're a Christian, do you believe in the virgin birth?"
I replied that the question itself would have been viewed as odd by our theological forefathers. The virgin birth is one of the cornerstones of Christian belief. Without it, you don't have Christianity. The original questioner accused me of "gatekeeping." She said, "You don't get to decide who's in and who's out." And she's right. I don't. We don't make orthodoxy. As Chesterton said, "It is making us!"
So you and I have the option, of course, of believing or disbelieving in the virgin birth. But if we disbelieve, we don't get to call our tailor-made theology "Christianity." Orthodoxy is not a buffet. The virgin birth is an essential tenet of Christianity -- a first-order doctrine -- and here is why:
1. The Scriptures are not ambiguous about Christ's birth.
Sometimes we hear theologians say the "virgin" referred to in Isaiah's messianic prophecy (7:14) is simply a "young woman of marriage-able age." The word of course does include that semantic possibility, but in the light of the new covenant, we see with a lot more clarity what "kind" of virgin the Christ child is born to -- not simply a young woman of marriage-able age, but a woman who had never experienced sexual intercourse.
"Now the birth of Jesus Christ took place in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child from the Holy Spirit." -- Matthew 1:18
"Before they came together" is a reference to sexual intercourse. Before Joseph and Mary had been married or engaged in any procreative activity, she was found to be with child. And if that reference isn't clear enough, Matthew tells us that the child is "from the Holy Spirit." Then, in verse 25, Matthew mentions that Joseph did not "know" Mary (in the sexual sense) until after she'd given birth to this son.
Luke 1 is even more clear. The angel tells Mary she will conceive a son. She asks how that's possible, since she is a virgin, which is an odd thing to say if she only means she's young. And the angel explains that the Holy Spirit will "overshadow" her (v.35).
2. Creedal Christianity maintains the virgin birth as an essential tenet of orthodoxy.
The earliest and longest-lasting belief of the historic Church has been that Christ was born of a woman who was a virgin when he was conceived. The oldest creed, of course, The Apostles Creed, plainly states that Jesus was "conceived by the Holy Spirit [and] born of the virgin Mary."
For roughly 2,000 years, then, the virgin birth has been a non-negotiable for true Christianity.
See also the Nicene Creed:
"I believe . . . in one Lord Jesus Christ...[who] for our salvation came down from heaven and was incarnate by the Holy Spirit of the virgin Mary and was made man."
And the Athanasian Creed:
"We believe and confess that our Lord Jesus Christ, the Son of God, is God and man; God of the substance of the Father, begotten before the worlds; and man of the substance of his mother, born in the world."
"We all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ . . . begotten, for us men and for our salvation, of Mary the Virgin, the God-bearer . . ."
This is just a sampling.
The creeds are of course not inerrant like the Scriptures or authoritative in the same sense as the Scriptures, but they are the roundly acknowledged borderlines for Christian orthodoxy by the Church catholic (universal). Fudging with creedal Christianity is fudging with Christianity.
3. Denying the virgin birth is inextricably connected to other Christological heresies.
The problem with trying to remove the virgin birth from one's Christian faith is that you inevitably discover a domino effect of subsequent errors arising from the denial. In his entry on the virgin birth in The Evangelical Dictionary of Theology, John Frame elucidates the critical doctrinal implications:
The deity of Christ. While we cannot say dogmatically that God could enter the world only through a virgin birth, surely the incarnation is a supernatural event if it is anything. To eliminate the supernatural from this event is inevitably to compromise the divine dimension of it.
The humanity of Christ. This was the important thing to Ignatius and the second century fathers. Jesus was really born; he really became one of us.
The sinlessness of Christ. If he were born of two human parents, it is very difficult to conceive how he could have been exempted from the guilt of Adam’s sin and become a new head to the human race. And it would seem only an arbitrary act of God that Jesus could be born without a sinful nature. Yet Jesus’ sinlessness as the new head of the human race and as the atoning lamb of God is absolutely vital to our salvation.
Indeed, it is difficult, pardon the pun, to conceive of the dual natures of Christ, the sinlessness of Christ, and the Incarnation proper itself generally without the logic of the virgin birth.
Further, the reasons for denying the virgin birth are basically the same reasons for rejecting orthodox Christianity altogether. If you reason by way of naturalism or materialism that a human life cannot be conceived miraculously apart from the presence of a male seed, what stops you from rejecting any of Christ's miracles, including his resurrection, or the powerful efficacy of the atoning cross itself? Christianity is dependent upon a theistic -- and thus, supernatural -- worldview, which opens up the door to all manner of inscrutable wonders. The Bible is full of them. Why pick and choose just one? It's like selecting one orange from the bottom of the pile in the produce aisle. Very quickly your feet are covered in oranges.
Thus, to tailor-make your Christianity without this or that doctrine is to tailor-make a different faith entirely. Denying the virgin birth may make more sense in our allegedly more enlightened age, but in doing so, you don't end up with a better Christianity. You just end up with Nestorianism or Monophysitism or Psilanthropism or Ebionitism or some other -ism that won't save you.