Short answer: the apostle Paul. Evangelical Christians overwhelmingly accept Paul as the human author of 1 Corinthians (including 1:18), writing by apostolic authority and inspiration.

Key facts and context
– The verse (KJV): “For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.”
– Where and when Paul wrote: 1 Corinthians was written during Paul’s ministry in the mid‑1st century (commonly dated about AD 53–55), traditionally from Ephesus, to the church at Corinth.
– Why Paul wrote: Corinth’s church faced divisions, moral failures, and misunderstandings about wisdom, boasting and the Christian life. Chapter 1 sets up Paul’s central point that Christ crucified — not human wisdom or status — is the foundation of true salvation.

Why evangelicals identify Paul as the author
– Internal evidence: the letter opens with Paul’s customary greeting (1 Cor 1:1–3) and contains personal details, travel plans, and references to people and events consistent with Acts and other Pauline letters.
– Early church testimony: early Christians and church fathers consistently attribute 1 Corinthians to Paul.
– Scholarly consensus (including evangelical scholarship) regards 1 Corinthians as authentically Pauline; there is little serious dispute about Paul’s authorship of this particular epistle.

Theological significance from an evangelical perspective
– Centrality of the cross: 1:18 sums up a core evangelical conviction — the cross is the decisive act of God for salvation. What looks like folly to the world is God’s saving power for those who trust Christ.
– Contrast of wisdom and power: Paul contrasts human “wisdom” and status with God’s wisdom revealed in Christ’s apparent weakness. Salvation is by God’s grace, not human achievement or rhetoric.
– Authority and application: evangelicals treat Paul’s teaching here as authoritative for doctrine (the nature of the gospel) and ethics (humility, unity, reliance on God rather than worldly wisdom or status).

Brief background on Paul (relevant to understanding his authority)
– Born Saul of Tarsus; a Jew (Hebrew of Hebrews), trained as a Pharisee under Gamaliel (Acts 22:3; Philippians 3:5).
– A Roman citizen and a tentmaker by trade (Acts 16:37–39; Acts 18:3).
– Persecuted the early church until his conversion on the Damascus road (Acts 9); afterward became the apostle to the Gentiles and planted many churches, including the one at Corinth.
– Authored several New Testament letters that shaped early Christian doctrine; evangelicals hold these letters to be inspired Scripture and normative for faith and life.

Practical application often drawn from 1:18
– Preaching and living the gospel even when it seems foolish by cultural standards.
– Rejecting pride in human wisdom, status, or power; trusting Christ’s cross as the means of salvation.
– Pursuing unity and humility in the church because the gospel makes all believers equal before God.

If you’d like, I can:
– Give a brief exegesis of the Greek behind “foolishness” and “power,”
– Explain how 1:18 connects to Paul’s later treatment of the cross in 1 Corinthians 2–3 and 1 Corinthians 15, or
– Summarize how major evangelical commentators (e.g., Fee, Wright from an evangelical perspective) interpret this verse.