Short answer: Evangelicals identify the author of Isaiah 7:14 as the prophet Isaiah (Isaiah son of Amoz), an eighth‑century BCE prophet in Judah. They read the verse as both a sign given in Isaiah’s day and—ultimately—a Messianic prophecy fulfilled in Jesus.

Key points an evangelical would emphasize

– Text (ESV rendering): “Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel.”
– Author and date: Isaiah son of Amoz, active in Judah ca. 740–700 BC during the reigns of Uzziah, Jotham, Ahaz and Hezekiah. Evangelicals typically affirm Isaiah as the human author of chapters 1–39 and, by theological conviction in prophetic inspiration, regard Isaiah’s words as authoritative prophecy.
– Historical context: The verse is set during the Syro‑Ephraimite crisis (Ahaz faced coalition pressure from Israel and Aram/Syria). Isaiah spoke to King Ahaz, offering a divine “sign” to reassure the king and the nation.
– Meaning of the key word: The Hebrew word translated “virgin” is almah. Evangelicals argue that almah can and does imply virginity in this context, and they note that the ancient Greek Septuagint translates it with parthenos (“virgin”), a reading adopted by Matthew (Matt. 1:22–23) when he cites this verse about Jesus.
– Fulfillment: Evangelicals commonly hold a “near and far” or dual‑fulfillment view: the sign had an immediate relevance in Isaiah’s day (a child born as a sign to Ahaz), but it also looked forward typologically and prophetically to the ultimate fulfillment in the birth of Jesus Christ—God with us (Immanuel).
– Theological significance: For evangelicals the verse is an explicit Old Testament prophecy pointing to the virgin birth and the incarnation—central to Christology. Matthew’s citation in the New Testament is taken as the authoritative interpretation by the early church and by evangelical theology.
– Response to critical scholarship: Some critical scholars propose later authorship for parts of Isaiah or argue almah simply means “young woman.” Evangelicals typically defend Isaiah’s authorship on internal and theological grounds and defend the traditional translation/meaning in light of the LXX and NT usage, as well as the doctrine of predictive prophecy.

Further reading (evangelical commentators)
– J. Alec Motyer, The Prophecy of Isaiah (IVP) — evangelical, classic exposition.
– John N. Oswalt, The Book of Isaiah, volumes 1–2 (NICOT; Eerdmans/IVP) — scholarly evangelical commentary.
– Walter C. Kaiser Jr., studies on Isaiah and prophecy (various shorter works).

If you’d like, I can summarize the arguments for and against the single‑author view in more detail, or show how Matthew uses the verse in the New Testament.