PaulThe earliest writing we have is Paul, who says nothing of a virgin birth, but then again says nothing of Jesus' life at all, so we cannot really draw any conclusions there.
MarkI think it is more significant that the virgin birth is missing from Mark, who, afterall, wrote a lot about Jesus' life. Mark is supposedly the account of Peter told through Mark. Peter was not there at the birth, so one might excuse Mark on this basis, but Peter was also not there at Jesus' baptism, and that is present in Mark.
The reason, as argued here, is that Mark believed Jesus was adopted the Son of God at the baptism. To Mark, Jesus' birth was of no great significance, and when Mark was writing it is likely that the story of a virgin birth had yet to appear in the Christian community.
None of the early texts mention a virgin birth because no one had heard of it at that time.
MatthewOkay, so Matthew. You probably know what I am going to say here already. Matthew tries to link Jesus to Old Testament prophecies numerous times. In Isaiah 7, as discussed here, two great kingdoms, Israel and Syria, are threatening Jerusalem, and Isaiah prophesies that both will soon fall - before the unborn Immanuel can tell right from wrong. That prophecy is the basis of the virgin birth, when you add in the LXX using the Greek for virgin, when the Hebrew said young woman.
It is very doubtful the author was the apostle Matthew (see here). Instead we have an anonymous author writing a polemic perhaps 90 years after the birth. The author of Matthew's gospel gets his evidence from the Old Testament - and a poor translation of it at that - and not people who were actually there.
The account in Luke is rather strange:
Luke 1:6 In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, 27 to a virgin betrothed[b] to a man whose name was Joseph, of the house of David. And the virgin's name was Mary. 28 And he came to her and said, “Greetings, O favored one, the Lord is with you!”[c] 29 But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. 30 And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. 31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. 32 He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, 33 and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”
34 And Mary said to the angel, “How will this be, since I am a virgin?”
Suppose we exclude that sentence from the account... Let us suppose she is not as dumb as a sack of hammers, and she had a clue about the birds and the bees. Is it possible the verse was added later, to agree with Matthew?
Without that one verse the virgin birth becomes less certain. The passage continues:
35 And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born[e] will be called holy—the Son of God. 36 And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. 37 For nothing will be impossible with God.” 38 And Mary said, “Behold, I am the servant[f] of the Lord; let it be to me according to your word.” And the angel departed from her.
I well may be wrong here, but the fact is that the gospel of Luke does not actually make Mary a virgin when Jesus was conceived; it only makes Mary look very naive about how babies are made.
Of course, the Gospel of Luke was written around the time the Gospel of Matthew was. Another likely scenario is that stories of a virgin birth were circulating in the Christian community sometime between Mark's gospel, and Matthew's and Luke's, and so both authors adopted it into their narratives.
ActsThere is no mention anywhere in Acts of the apostles talking of Jesus' virgin birth. They cite all sorts of evidence of Jesus' significance to the crowds they preach to, but none of them mention the virgin birth.
Perhaps because none of the disciples had heard of it.
Why Did Mary Claim A Virgin Conception?The simple answer is that she never did. The virgin birth story probably only circulated after she was dead, some time after Mark's Gospel was written.
Jesus might have been born out of wedlock, but if we believe the nativities, she was already betrothed, and in that culture betrothal was as big a deal as marriage. I am not aware of anything in the OT that prohibits sex between a man and woman once they are betrothed (and the story of Ruth gives a precedent even outside of betrothal). If it was not forbidden, it is sure to have happened, and if it happened, there are sure to be times when the girl ended up pregnant.
Mary's Reaction to Jesus' MinistryThe evidence for the virgin birth is therefore pretty slim. Is there any evidence against it? The best evidence is from the Bible itself.
Mark 3:21 When his family heard about this, they went to take charge of him, for they said, "He is out of his mind."According to Christian doctrine, an angel appeared to Mary saying Jesus was the Son of God, and she conceived him whilst still a virgin. And yet when Jesus is healing people and casting out demons, she somehow forgets that, and declares he is mad!
Remember, this was supposedly the woman who was selected for her faith,and yet despite the appearance of an angel and having a virgin birth, she still does not believe Jesus is the messiah!
Perhaps we should hail Mary as the greatest skeptic ever...