One of the most hotly-contested biblical concepts today (and by "today" I include "at this time of year") is the doctrine of the Virgin Birth. We hold that Jesus was born of a virgin (Isa 7:14; Luke 1:27), that He had no earthly father but was produced by the Holy Spirit (Matt 1:20; Luke 1:35) impregnating Mary who had "never known a man" (Luke 1:34). This, of course, is outside of the realm of "normal". I mean, no human being actually comes into being that way. Ergo, clearly, the story is not true. And, look, it is certainly tradition, but can't we just all agree to disagree on this one? Does it really matter in the overall scheme of Christianity? One of the primary schisms between conservative Christianity and liberal Christianity was specifically this point -- the Virgin Birth. But is it really that important?
First, let's keep the question clear. We're talking about virgin conception, not anything beyond that. "Virgin Birth" is a bit misleading in that regard. Second, we're not talking about either the "Immaculate Conception" -- the claim that Mary was sinless, tacitly impossible since she referred to Jesus as her "Savior" (Luke 1:47) -- or the perpetual virginity of Mary, a second Roman Catholic (but not biblical) doctrine. Neither is in view here. Just whether or not Jesus was born of a woman who had never had sexual relations with anyone, but was fathered by God. Just that far.
Detractors (starting from the position of denying miracles because if miracles are possible, there is no problem with this one), argue that "virgin" (Isa 7:14; Matt 1:23) should be translated "maiden" and that all that "virgin" stuff isn't necessary. The fact that "maiden" referred to an unmarried woman who was expected to be a virgin is irrelevant. Clearly the disciples simply worked in a "virgin birth" because that was so popular at the time among the followers of pagan deities and, hey, if they can have a god of virgin birth, we can, too. Never mind that the whole Mithras connection is a fabrication. Still, can't we just get along? How important is this Virgin Birth thing?
It is, as it turns out, not peripheral. It is essential.
1. The Bible teaches it. Included in that, the authors of the Gospels believed it. Beyond and because of this, the Church has always believed it. Never, prior to the Enlightenment, was there a question regarding that particular claim of Scripture. Thus, if there was no Virgin Birth, then the Bible is not reliable and the Holy Spirit failed to lead His own into all truth until the 19th century or so.
2. The Virgin Birth is necessary for our understanding of Christ. First, it is only in this Divine-Father-and-human-mother scenario that we can get to the biblical identity of Christ as both human and divine. If He was the product of human father and mother, He was only human. Second, the Virgin Birth is essential to explain how Christ did not inherit the sin nature. Scripture teaches that He "in every respect has been tempted as we are, yet without sin." (Heb 4:15) No other human can make that claim because all other humans are born with a sin nature (Rom 5:12; Psa 51:5).
Consider the alternative. Yes, Scripture refers to the Virgin Birth, but let's say it never happened. As it turns out, then, Mary was a ... sexually immoral woman, either with Joseph or another man. Jesus was born out of wedlock and the Pharisees' accusation of Him being an illegitimate child (John 8:41) was valid. Jesus was a product of sin, complete with a sin nature. If He managed to live out a sinless life, then it is a distinct possibility than any other person could do the same. However, that "if" would be a massive one and we'd likely have to actually rule it out because 1) no one else has ever done it and 2) the Bible says it can't happen. Well, now, we're starting to run into real problems here. Apparently the Bible is not reliable. After all, it claims the Virgin Birth and, yet, we've ruled it out. The Bible says that Jesus was sinless but that's not likely, either. And, oh, by the way, if He was not sinless, He is also ruled out as ... wait for it ... Savior (Heb 7:27). So, if the Virgin Birth is a myth, we have a failed Holy Spirit who could not lead us into all truth for thousands of years, an unreliable "Word of God", and, ultimately, a non-Savior.
Given the impact if the Virgin Birth is a lie, it is not a side issue. Jesus's divinity, His sinlessness, His ability to save, and the reliability of both the Bible and the Holy Spirit all hinge on this point. That makes it a fundamental issue.