Short answer
– The evangelical consensus is that the author of Isaiah 9:6 is the prophet Isaiah (Isaiah son of Amoz), an 8th‑century BC prophet active in Judah. Evangelicals generally treat the book of Isaiah — certainly chapters 1–39, which include ch. 9 — as the genuine work of the historical Isaiah.

Who Isaiah was (evangelical summary)
– Name and date: Isaiah (Hebrew: Yeshayahu, “Yahweh saves”), ministered about 740–680 BC during the later years of Uzziah and the reigns of Jotham, Ahaz and Hezekiah.
– Location and role: He was a prophet in Jerusalem called to speak to the kings and the people of Judah. He addressed issues of idolatry, social injustice, covenant unfaithfulness, and the national threat from Assyria.
– Calling and character: Isaiah’s prophetic call is recorded in Isaiah 6 — a dramatic vision of God’s holiness and a commissioning to speak God’s word. He is remembered for moral courage in confronting rulers and calling the nation to repentance.
– Family detail: The book refers to “the prophetess” and a son (Isaiah 8:3–4; ch. 7 and 8), suggesting a family life; Jewish tradition later says he was martyred under Manasseh (not a biblical account, but part of later tradition).

Why evangelicals attribute Isaiah 9:6 to Isaiah
– Internal biblical evidence: Isaiah 9:6 is in the first section of the book (chapters 1–39), which traditionally is the work of the historical Isaiah.
– New Testament practice and Christian tradition: Jesus and the NT writers repeatedly quote and treat Isaiah as authoritative prophetic scripture; the church historically read Isaiah’s prophecies as messianic, a practice evangelicals continue.
– Consistency of style and theology: Evangelicals argue the themes and theological emphases in Isa. 1–39 fit Isaiah’s historical setting and prophetic calling.

Context and significance of Isaiah 9:6 (evangelical reading)
– Immediate context: Isaiah 9 speaks of light breaking into darkness amid judgment and national crisis (Assyrian threat). It brings a promise of deliverance and a ruling figure from David’s line who will establish righteous government.
– Isa. 9:6 (KJV): “For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder….” Evangelicals read this both as a sign in Isaiah’s day (a child born bringing hope) and as a prophecy pointing ultimately to the Messiah, Jesus Christ.
– Messianic titles: The names/titles in many translations — “Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace” (or similar renderings) — are understood by evangelicals to express the Messiah’s unique, divine and eternal character. Because the titles imply deity and eternal rule, Isaiah 9:6 is regarded as a significant Old Testament prediction of the incarnate Lord Jesus.
– Dual/“near and far” fulfillment: Many evangelicals hold a dual application — an immediate assurance to Isaiah’s contemporaries (e.g., hope for Davidic continuity in the face of crisis) and a fuller, ultimate fulfillment in Christ.

Practical implications for evangelical faith
– Authority of prophecy: Isaiah is seen as a divinely inspired prophet whose words point forward to God’s redemptive work.
– Christ-centered reading: Isaiah 9:6 is frequently used in evangelical preaching and worship (especially at Christmas) to affirm Christ’s deity, kingship, and peace-giving reign.
– Call to trust and holiness: Like much of Isaiah, the verse frames hope in God’s sovereign plan and calls people back to covenant faithfulness.

A brief caveat
– Critical scholarship often divides Isaiah into multiple authors (Deutero- and Trito-Isaiah) and dates parts of the book later. Evangelicals typically affirm Isaiah’s historical authorship of chapters 1–39 (including ch. 9), while recognizing that serious scholarship raises questions worth studying. Evangelicals usually defend Isaiah’s authorship on grounds of internal coherence, early Jewish/Christian use, and theological continuity.

If you’d like, I can:
– Provide a short devotional reflection on Isaiah 9:6 from an evangelical perspective.
– Summarize how the New Testament connects Isaiah’s prophecies to Jesus.
– Point to evangelical commentaries or resources for deeper study.