Pentecostalism is the newest of the major Christian traditions and the fastest growing, and its founding claim is unusually specific.
The doctrinal move was made first by Charles Parham, who taught at a small Bible school in Topeka in 1901 that speaking in tongues is the initial evidence of a baptism in the Holy Spirit distinct from and subsequent to conversion. That precise formulation, not that tongues occur, which many revivals have reported, but that tongues are the sign that a particular thing has happened, is what makes classical Pentecostalism a distinct movement rather than an intensity of feeling.
The revival that carried it began in Los Angeles in 1906 under William J. Seymour, a one-eyed Black preacher and son of former slaves, who had been permitted to hear Parham's teaching only from outside the classroom door under segregation law. The Azusa Street meetings ran for about three years, were extensively reported and mocked in the press, and were racially integrated at a moment when almost nothing in American religion was. Missionaries left them for dozens of countries within months.
The theological ground had been prepared by the Holiness movement, itself an outgrowth of Wesley's entire sanctification. Holiness teaching had already established the expectation of a definite second work of grace after conversion; Pentecostalism identified that experience with the Spirit's baptism and attached a required sign to it.
Historians differ on how much weight to give Azusa Street against the many near-simultaneous revivals elsewhere, in Wales in 1904, in India, in Korea in 1907, and recent scholarship has moved toward a picture of multiple origins rather than a single fountainhead. The Azusa account remains the movement's own.