Does the New Testament directly claim that Mary was a virgin at the time she conceived Jesus, or is the virgin birth of Christ a quaint tradition, not rooted in any theological truth? Perhaps we should double-check the accounts of the birth of Christ in both Matthew and Luke’s Gospel.
Matthew the Evangelist tells us that Mary was found to be with child of the Holy Spirit; and her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly (Mt 1:18-24). Only when an angel intervened to inform Joseph that Mary conceived by the Holy Spirit, did Joseph resolve to stay with her.
In chapter one of Luke’s Gospel, the third evangelist speaks even more directly of Mary’s status at the time she conceived Jesus. Luke refers to Mary as παρθένος (virgin) twice in verses 27-28 (original Greek translation here). According to Luke, the angel Gabriel then tells Mary that she will bear a son, whom she shall call Jesus. He will be great, and will be called the Son of the Most High.. . Then, for a third and fourth time in Luke chapter 1, (and just in case we did not get the message) the evangelist hammers away at the point that Mary is a virgin. For instance, Mary responds to the angel’s news that she will bear a son with some skepticism, saying, how shall this be, since I have no husband? And the angel Gabriel responds, the Holy Spirit will come upon you. (Luke 1:27-35)
In short, the answer is, “yes,”- Scripture does indeed tell us that Mary was a virgin, who conceived the child Jesus by the Holy Spirit.  The theological point being that Jesus is not entirely “of man:” he is also ‘of God.’ Finally, Matthew 1:23 tells us that Mary’s virgin birth fulfills an Old Testament prophecy:
Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. (Isaiah 7:14 KJV)
διὰ τοῦτο δώσει κύριος αὐτὸς ὑμῖν σημεῖον ἰδοὺ ἡ παρθένος ἐνγαστρὶ
ἕξει καὶ τέξεται υἱόν καὶ καλέσεις τὸ ὄνομα αὐτοῦ Eμμανουηλ
לָ֠כֵן יִתֵּ֨ן אֲדֹנָ֥י ה֛וּא לָכֶ֖ם א֑וֹת הִנֵּ֣ה הָעַלְמָ֗ה הָרָה֙ וְיֹלֶ֣דֶת בֵּ֔ן וְקָרָ֥את שְׁמ֖וֹ עִמָּ֥נוּ אֵֽל׃ חֶמְאָ֥ה וּדְבַ֖שׁ יֹאכֵ֑ל לְדַעְתּ֛וֹ מָא֥וֹס בָּרָ֖ע וּבָח֥וֹר בַּטּֽוֹב׃
Some scripture scholars have argued that the translation from Hebrew to English is inexact, since the Hebrew term “almah” in Isaiah 7:14 implies “young woman,” rather than “virgin.”
This position has its critics, though, even among Protestant scholars. Many Protestant translators and biblical scholars, including those who authored the King James, the New King James, the New International, the New American Standard, and the Third Millenium versions of the bible, all translate “almah” as “virgin.”
Perhaps this is because the Greek version of Hebrew scripture – the Pentateuch – translates “almah” into Greek as “parthenos.” Parthenos means “virgin,” confirming that Jewish scholars, two hundred years before the birth of Christ, interpreted Isaiah 7:14 as stating behold, a virgin shall conceive a son.
To summarize, suggesting that the Old Testament authors never intended the term “almah” in Isaiah 7:14 to be translated as “virgin” is quite inaccurate, since Jewish scholars were the first to chose the term “parthenos” (virgin) to convey the intent of Isaiah in verse 7:14, when they translated the Tanakh into Greek.
Here is the Sunday reading for the third Sunday of Advent:
This is how the birth of Jesus Christ came about…
When his mother Mary was betrothed to Joseph,
but before they lived together,
she was found with child through the Holy Spirit.
Joseph her husband, since he was a righteous man,
yet unwilling to expose her to shame,
decided to divorce her quietly.
Such was his intention when, behold,
the angel of the Lord appeared to him in a dream and said,
“Joseph, son of David, do not be afraid to take Mary your wife into your home.
For it is through the Holy Spirit
that this child has been conceived in her.
She will bear a son and you are to name him Jesus,
because he will save his people from their sins.”
All this took place to fulfill what the Lord had said through the prophet:
Behold, the virgin shall conceive and bear a son,
and they shall name him Emmanuel,
which means “God is with us.”
When Joseph awoke,
he did as the angel of the Lord had commanded him
and took his wife into his home.
 John Paul II.