A claim of a virginal conception in ancient history isn't something that can be directly examined. We make judgments based on more indirect criteria (whether the supernatural is possible, the genre of the accounts, the sources available to the authors, etc.). In earlier posts, I've argued for some of the evidence that indirectly supports the Christian belief that Jesus was conceived of a virgin, such as the earliness of the gospel accounts, their genre, the availability of relevant sources, and the Divine inspiration of scripture. What I want to do in this post is address some common arguments that are raised against the doctrine.
People often compare the Christian virgin birth accounts to unhistorical birth accounts in Jewish or pagan sources. However:
"Yet most alleged parallels to the virgin birth (see Allen 1977: 19; Soares Prabhu 1976: 5-6; cf. Grant 1986: 64) are hopelessly distant, at best representing supernatural births of some kind (Barrett 1966: 6-10; Brown 1977: 522-23; Davies and Allison 1988: 214-15; Hagner 1993: 17; even further are ancient biological views, e.g., Arist. Gen. An. 3.6.5; Ep. Arist. 165). Certainly pagan stories of divine impregnation, which typically involve seduction (e.g., Ovid Metam. 3.260-61) or rape (Ovid Metam. 3.1-2), bear no resemblance to a virgin birth. Even most proposed Jewish parallels (Daube 1973: 6-9; cf. also 2 Enoch 71; Gen. Rab. 53:6) are too late or on closer examination have little merit (cf. Brown 1977: 523-24); Philo’s claims that God supernaturally opened wombs (Schweizer 1975: 33; cf. Vermes 1973: 220) probably simply imply that only God can provide conception (cf. Gen 30:2; cf. Meier 1991a: 221-22)." (Craig Keener, A Commentary On The Gospel Of Matthew [Grand Rapids, Michigan: William B. Eerdmans Publishing Company, 1999], pp. 83-84)
Ben Witherington comments that "most scholars" think that the infancy narratives are more like Jewish infancy accounts than pagan birth legends (in Joel B. Green, et al., editors, Dictionary Of Jesus And The Gospels [Downers Grove, Illinois: InterVarsity Press, 1992], p. 60). Darrell Bock writes that there’s a "consensus" among scholars to reject the view that the virgin birth was derived from pagan mythology (Luke, Volume 1, 1:1-9:50 [Grand Rapids, Michigan: Baker Books, 1994], n. 4 on p. 103). Bock also makes another relevant point. If the concept of a virgin birth was so important to the Christians that multiple gospel authors would be willing to fabricate an account, then why is it that "the virgin birth plays only a minor role in Luke and is largely absent from the writings of the early church and the church fathers" (ibid., p. 112)? Ben Witherington explains:
"It is doubtful that the idea of a virginal conception was part of Jewish messianic expectations in or before the era when the Gospels were written...It is difficult if not impossible to explain why Christians would create so many problems for themselves and invite the charge of Jesus' illegitimate birth by promulgating such an idea if it had no historical basis." (in Joel B. Green, et al., editors, Dictionary Of Jesus And The Gospels [Downers Grove, Illinois: InterVarsity Press, 1992], p. 70)
In the second century, Justin Martyr commented:
"Now it is evident to all, that in the race of Abraham according to the flesh no one has been born of a virgin, or is said to have been born of a virgin, save this our Christ." (Dialogue With Trypho, 66)
It doesn't seem that a virgin birth was a common Messianic expectation or a common claim. And the idea that Christianity borrowed the concept from paganism is unlikely even aside from the differences mentioned by Craig Keener above. Christianity arose in a highly anti-pagan atmosphere, and the alleged pagan parallels are of too vague a nature to prove what critics want to prove. The doctrine of the virgin birth originated in a highly Jewish religion that was centered in Israel. The earliest Christians believed that their gospel was "to the Jew first" (Romans 1:16). They considered the Jewish people their "fathers" (Romans 9:5). They viewed pagan religion as a system of "ignorance" (Acts 17:23) and "foolishness" (Romans 1:22-23). Pagan gods were "no gods" (Galatians 4:8). Pagan religions were viewed as demonic (1 Corinthians 10:14-22). Pagan religions left people "dead in trespasses and sins" and "without God and without hope" (Ephesians 2:1, 2:12). The infancy narratives are written in a highly Jewish context, with many citations of Old Testament scripture, references to Jewish tradition, Hebraisms, etc. Since conception by means of intercourse with the pagan gods was common in pagan mythology, the Christian concept of a virgin birth is more anti-pagan than a parallel to paganism.
The best explanation for why the early Christians claimed a virgin birth is what Luke suggests in the opening verses of his gospel. They believed that it happened, even though the Messiah wasn't commonly expected to be born of a virgin and even though a virgin birth wouldn't do much to appeal to the Gentile world. Celsus, a second century critic of Christianity, rejects the virgin birth account, as we would expect, but attributes the virgin birth claim to Jesus Himself (Origen, Against Celsus, 1:28). The timing of the gospels and their sources suggests that the virgin birth claim was circulating when close relatives of Jesus were still alive. The claim may have been widely circulating even prior to Jesus' death, as Celsus suggested.