The controversy surrounds the translation of Isaiah 7:14 "Therefore the Lord himself shall give you a sign; behold, a virgin shall conceive, and bear a son, and shall call his name Emmanuel." This Old Testament prophecy is quoted in the gospel of Matthew (Mt 1:23) and specifically applied to the virginal conception of Christ.
Christians have always cherished this prophecy of Isaiah and its miraculous fulfillment in the virgin birth of Jesus the Messiah. Likewise, non-believers have attacked this prophecy in an attempt to discredit Christ and his Church; the attack is a weak one that is easily refuted.
The Hebrew word translated as virgin, almah, can also be translated as "young woman" but as Strong's Hebrew Lexicon notes "there is no instance where it can be proved that almah designates a young woman who is not a virgin."
Additional evidence that the correct translation is "virgin" is supplied by the Septuagint version of the Bible, a Greek translation of the Old Testament made several centuries before Christ. It was translated by Jewish scholars for use by Greek-speaking Jews, mainly in Alexandria.
The Septuagint translates the Hebrew almah into Greek as parthenos. This Greek term has the precise meaning of "virgin." So several centuries before the birth of Christ, before there was any reason to attack his Church, the meaning of Isaiah 7:14 was clear: almah = parthenos = virgin.