A class of first-century Gentiles who had attached themselves to the Jewish synagogue — attending worship, observing some Jewish ethical practices, typically believing in the God of Israel — without undergoing full proselyte conversion (which required circumcision for men and full Torah observance). Greek phoboumenoi ton theon ("those fearing God") or sebomenoi ton theon ("those reverencing God"). The term appears a dozen times in Acts, especially in contexts involving synagogue evangelism (Acts 10:2 — Cornelius; 13:16, 26 — Paul in Pisidian Antioch; 17:4 — Thessalonica).
God-fearers were the evangelistic bridge population of the early Church. They were already sympathetic to the God of Israel, already reading the Hebrew Scriptures in synagogue, already praying and practicing monotheism — but many had stopped short of conversion because of circumcision, dietary laws, and full Torah observance. When Paul walked into a synagogue and preached Jesus as the promised Messiah whose death and resurrection opened the covenant to all nations without requiring Jewish ceremonial law, the God-fearers were the first to believe in droves. Cornelius the centurion (Acts 10) is the textbook case — already devout, prayerful, generous, but incomplete until Peter arrived with the gospel. The explosive growth of the Gentile Church in Paul's first decade of ministry rides in large part on God-fearers converted from the synagogue. The modern equivalent is everywhere: the cultural Christian, the spiritual-but-not-religious seeker, the sympathetic unbeliever who goes to church but has never owned Christ. God-fearers are a massive harvest field. The Cornelius event also settled the question that would rock the early Church (Acts 15): Gentiles are welcomed in by faith, not by becoming Jews first.