Gospel of Matthew
Matthew's use of Isaiah
The Gospel of Matthew presents Jesus's ministry as largely the fulfilment of prophecies from Isaiah, but in the time of Jesus the Jews of Palestine no longer spoke Hebrew, and Isaiah had to be translated into Greek and Aramaic, the two commonly used languages. The Greek translation, the Septuagint, mistranslated the word almah, meaning a young woman of childbearing age who had not yet given birth, as parthenos, which means virgin. This gave the author of Matthew the opportunity to interpret Jesus as the fulfilment of prophecy: he makes Jesus Immanuel, God is with us (Matthew 1:23), the divine representative on earth, and underlines Jesus' status as "Son of God" by asserting that Joseph did not have sexual intercourse with Mary before she gave birth (Matthew 1:25).
Revised Standard Version
When the Revised Standard Version translators rendered "almah" as "young woman" in 1952 it immediately became a center of controversy for conservative Christians, who believed that this passage predicted the virgin birth of Jesus. The RSV quickly replaced the KJV in many churches across America, but fundamentalist American Christians argued that nowhere in the Old Testament was an almah anything other than a young unmarried girl, and one pastor publicly burned a copy of the RSV. Isaiah 7:14 became a litmus test of orthodoxy among conservatives, but most modern Bible translations use "young woman".
On his blog, New Testament scholar Bart D. Ehrman has argued that the original meaning of the word parthenos in the Septuagint (i.e., the Hebrew Bible translated by Hellenistic Jews in Koine Greek) is "young woman", not "virgin", but the word changed meaning over the centuries; thus the authors of Matthew and Luke believed instead that Isaiah had predicted a virgin birth for the coming Messiah, using the common understanding of the term in their time.