NB. Jesus didn't take on 'David's line' through Joseph being his 'adoptive father' as Rom l:3 makes quite clear, ie. "descended from David according to the flesh".
So there is a problem; Jesus was either of David's line - but that means he wasn't virgin born (ie. Joseph having to have been responsible for his conception), or he was virgin born, but that precludes him being of the Davidic line (because only Mary was involved in his humanity and she was not of the Davidic line) - so he couldn't have been the Messiah/Christ as the New Testament teaches.
The virgin-birth story is only found in two of the twenty-seven New Testament writings, and in Luke, the style of writing indicates the part that relates the story, was written after the following 22 chapters by a different author, and added on to the beginning of Luke afterwards. Furthermore the Catholic Jerusalem Bible admits that Matt most likely had its virgin birth story added to it also. In fact Luke conflicts sharply with Matthew, eg. (i) Luke has the birth in the time of the governor Quirinius (Luke 2:2, 3-7), whilst Matthew has it in the time of Herod, but the rule of these two never coincided or overlapped. The Christian "explanation" for this involving the Ramsay inscription regarding Quirinius as dummvir, is futile.
Both Luke and Matt have other major differences, eg. Matthew says the family fled from Judea immediately to Egypt after the birth (2:4-l4) to avoid Herod and stayed there until he was dead and even on returning, they avoided Judea in the south. However according to Luke, after the birth, the family calm went to Jerusalem in Judea and then up to Galilee (2:21-22,39). It is worthwhile noting that the only census known about (Luke has the journey to Bethlehem because of this) as one in 6 AD. Long after Herod died, and indeed long after Christians claim Jesus lived.
The only reason that Matthew's author seems to have the virgin-birth story is because he misunderstood an O.T statement (Isaiah 7:l4) that he read as messianic (which it isn't) and referring to a virgin birth (which it doesn't). With regard to Isa 7:l4, it is simply the story of Isaiah saying to king Ahaz of Judah that by the time that a young girl had conceived and her baby was born, the present threat from Syria would be over - 7:l4-17. There is NOTHING messianic about it at all.
As, in this, the child was to be called Emmanuel which means 'God with us', but the name 'Jesus' (actually, this is Greek for the Hebrew Jehoshua) means 'Yahweh is salvation', Jesus was therefore not called by the name Emmanuel and did not fulfil this 'prophesy'; however Matt's author misunderstood this. As Isa 8:3-4 says how Isaiah went immediately and impregnated his wife, and the prophesy is again made saying that before the child could even talk, Syria would by smashed by Assyria, it appears the Isa 7:l4 prophesy relates to Isaiah's own wife/child and does not have any messianic connotations.
In reality there is nothing miraculous in Isaiah's saying; he is only saying a woman (or in the Greek - a virgin) would conceive. It doesn't take too much to realise what has to happen for a virgin to conceive a child. He doesn't say that a girl who would give birth to the child would still be a virgin after conception. The author of Matthew was using the Septuagint 'LXX' - the Greek version of the Hebrew Bible compiled in the second century BC for the Greek-speaking Jews of the Diaspora - ie. living outside Palestine. It is generally accepted that some parts are very good, but in others it is faulty, eg. Daniel is such a poor translation in the LXX, the Jews wouldn't even use it.
The Hebrew word in Isa 7:l4 for the woman/'virgin' is "almah" and means NOT a virgin, but a young woman; it is in the LXX that it is rendered 'virgin' and there is the additional fact that in the Greek the root doesn't even necessarily mean a girl who has not had sex, but 'denotes fullness or the like - fully developed". The word actually used here has nothing to do with the virgin state.
As the Gospel writers used the LXX, they could not have been Palestinian Jews (ie. the apostles as stated in the Gospels) or they would have obviously used the Hebrew text and not made such errors.
Matt's author couldn't have been the apostle of that name as he wasn't a Palestinian Jew (nor either an eyewitness as he had to use Mark as a source to write his Gospel). He also makes other errors, eg. in 27: 9-l0 he says he is quoting Jeremiah but in fact he's quoting Zechariah ll:l2-l3. It is very apparent that the Gospel writers were NOT Palestinian Jews, but either Jewish Christians of the Diaspora or Gentile-Christians. In the case of Mark's author there has to be doubt whether he had even set foot in Palestine in view of the historical, chronological, geographical and theological errors he makes about first cent. Palestine.
But this is where it continues to be manifestly absurd. Jesus was supposedly a true Jew - a direct descendent of Abraham through David (Matthew 1), the Jewish Messiah, the Son of David (Matt 21:9), the 'lion of the tribe of Judah' (Rev 5:5) and yet whenever he quoted the Old Testament, according to the Gospels that is, he quoted the GREEK LXX version ! Furthermore, in some cases the Hebrew original of the LXX text he is quoting would not support the argument he is making, ie. because of the LXX's inaccuracies. In Mark 7:l-23 Jesus does this; although it would seem the LXX would support the point Jesus is making to the Pharisees, the Hebrew original in fact would not.
So we are asked to believe that Jesus - a true Hebrew Jew - chose to use the Greek translation of the Old Test. and furthermore, was unaware of the fact that he was using a passage that in reality was faulty and in the original would say something completely different, and be quite inappropriate for his argument, but according to the Gospels, he floored his orthodox Jewish opponents with this - a mistranslation of their own scriptures - and they did not challenge this ! The same applies with James (supposedly Jesus' brother and leader of the Jewish-Christian community in Jerusalem) in Acts 15 - he uses the LXX to support his argument, although the Hebrew original says something quite different and would not support his argument, and yet all the Jews in the audience didn't comment on this !!! Obviously as the writers of the Gospels & Acts were not Palestinian Hebrew-speaking Jews, they had to use the LXX but didn't realise the errors they were making.
Therefore, the bad news is that firstly the virgin birth is disproved by the Bible itself, and secondly, there is no written eyewitness testimony for Jesus' supposed life.
The situation is adequately summed up by Professor Fuller, Professor of New Testament, Union Theological Seminary, New York. (A Critical Introduction to the New Testament):
"Of the 27 books of the New Testament only the authentic Pauline epistles are, strictly speaking, the testimony of an apostolic witness. And even Paul...was not a witness of the historical Jesus.
Since the earliest witnesses wrote nothing...there is not a single book in the New Testament which is the direct work of an eyewitness of the historical Jesus..." (page 197).