Short answer
– Most evangelicals identify the author of the Epistle of James (hence James 1:12) as James the Just — the brother of Jesus and leader of the Jerusalem church.

Why evangelicals say this (key points)
– New Testament identification: Paul refers to “James” as one he met in Jerusalem and calls him a pillar (Galatians 1:19; Galatians 2:9). Acts portrays a prominent James as the leader at the Jerusalem council (Acts 15:13–21).
– Early church testimony: Second‑century witnesses and historians (e.g., Hegesippus as quoted by Eusebius, Clement of Alexandria, Origen) attribute the letter to James the brother of the Lord (often called “James the Just”).
– Fit with the letter’s character: The epistle’s strong Jewish‑Christian tone, practical moral teaching, concern for community life, and familiarity with Palestinian Jewish thought suit a Jerusalem leader who ministered to Jewish Christians.
– Linguistic and literary notes: The Greek is polished but shows Semitic rhythm and thought, which many evangelicals take as consistent with an Aramaic‑background leader composing (or dictating) a letter in Greek for a Jewish audience.
– Date and place: Evangelical scholars commonly date James early — often in the 40s–60s AD (frequently before or around the Jerusalem council), with composition in Palestine/Jerusalem where James ministered.

Relevant biblical markers
– Acts 12:2 records the martyrdom of James the son of Zebedee (so that James cannot be that apostle if the letter was written later). The James of Acts 15 and Galatians is usually identified with Jesus’ brother.
– Galatians 1:19 explicitly calls James “the Lord’s brother,” which supports identification with the Jerusalem leader.
– The letter’s addressees (“the twelve tribes in the Dispersion” — James 1:1) point to a Jewish audience, matching James the Just’s sphere.

Alternate views (and how evangelicals respond)
– Some modern critical scholars argue for a later date or pseudonymous authorship (a later writer using James’ name). Evangelicals generally respond that strong early attestation, the letter’s theological consistency with apostolic teaching, and its Palestinian/Jewish character favor authentic Jamesan authorship.

Pastoral note
– From an evangelical perspective, knowing James as the author helps read the letter as authoritative, apostolic teaching coming from a firsthand leader of the early church — practical instruction for faith under trial (James 1:12 is a classic example of that pastoral emphasis).

If you want, I can provide a short biography of James the Just from the New Testament and early sources, or summarize how James 1:12 fits his pastoral concerns.