Short answer
– The most likely author of Isaiah 1:16–17, from a conservative evangelical perspective, is the prophet Isaiah son of Amoz — the eighth‑century BC prophet active in Judah during the reigns of Uzziah, Jotham, Ahaz and Hezekiah.

Who Isaiah was (evangelical summary)
– Isaiah is presented in the Bible as a historical prophet called by God (see Isaiah 6). He likely belonged to the Jerusalem prophetic circles and appears to have had access to the royal court (he addresses kings and national issues).
– Biblical markers: the book is titled for Isaiah in Jewish and Christian tradition; the narrative voice in early chapters places the material in an 8th‑century BC Judahite setting (references to the four kings above; historical events and Assyrian threat).
– Isaiah’s family details are briefly mentioned in the book itself (e.g., Isaiah 7–8 refers to his children; Isaiah 6 records his prophetic commissioning).

Why evangelicals conclude Isaiah wrote Isaiah 1:16–17
– Traditional authorship: Jewish and Christian tradition from the earliest centuries attributes the whole book to Isaiah son of Amoz.
– Internal continuity: conservative scholars note that chapters 1–39 read as a unit tied to 8th‑century circumstances and the prophet’s direct ministry; Isaiah 1 is part of that core material.
– Manuscript evidence: ancient manuscripts (including the Great Isaiah Scroll from Qumran, 1QIsaª, dated around the 2nd century BC) testify to the book’s early existence and transmission as a single book named Isaiah.
– The evangelical conviction also rests on the doctrine of biblical inspiration: Isaiah’s prophetic role and the book’s place in the canon support single‑prophet authorship for the opening material.

Meaning and context of Isaiah 1:16–17 (brief exposition)
– The verses read as a covenant call to genuine repentance: religious ritual without moral reform is rejected. The language moves from ceremonial cleansing (“wash yourselves; make yourselves clean”) to ethical action (“learn to do good; seek justice; correct oppression”).
– This theme is typical for Isaiah: holiness before God, social justice, care for the vulnerable (widows, orphans), and the demand that outward worship be matched by right living.
– Historically, Isaiah rebuked Judah for religious formalism and social injustice in the face of national crisis (the Assyrian threat), calling people and leaders to faithful covenant obedience.

Pastoral and theological significance for evangelicals
– Moral-ethical demand: The passage is used to teach that true repentance involves changed behavior — not merely ritual or words.
– Social justice as biblical: evangelicals often point to Isaiah 1:16–17 as a biblical basis for concern for the oppressed, vulnerable, and for public justice as part of faithful living.
– Christological horizon: While this specific passage is an ethical summons, evangelicals read all prophetic witness in the light of Christ — seeing the call to purification and justice ultimately fulfilled in Jesus’ work and the transformed life of his followers.

A word on critical scholarship
– Some modern critical scholars argue for multiple authors or later additions in Isaiah (e.g., dividing the book into First, Second, Third Isaiah). Evangelicals who affirm single‑authorship respond with arguments from internal unity, historical markers, continuity of themes and the early canonical tradition. Most evangelicals accept that Isaiah 1 (and the surrounding material) belongs to the historical prophet Isaiah of the 8th century BC.

If you want, I can:
– Give a short expository paragraph on Isaiah 1:16–17 suitable for a sermon or study group.
– Summarize the main points of disagreement between conservative and critical scholars about Isaiah’s authorship.