The Pentecostal movement burst onto the religious landscape during the 20th century as a major force within Christianity. Its adherents, scattered across many churches and denominations, came to number over half a billion worldwide, suddenly making it a Christian tradition second in size and scope only to the Roman Catholic Church.

Some historians date the origins of the contemporary Pentecostal movement to January 1, 1901, when Agnes Ozman, under the teaching of Methodist preacher Charles F. Parham, "spoke in tongues" (glossolalia) at Bethel Bible School in Topeka, Kansas.

This particular event convinced many that the supernatural gifts and powers associated with the coming of the Holy Spirit at Pentecost and with the ministries of the early church in the book of Acts are still readily available to ordinary Christians who sincerely seek them.

Similar teachings and manifestations gained wide attention from 1906 to 1913 during the Azusa Street Revival at the Apostolic Faith Mission in Los Angeles. William J. Seymour, an African-American Holiness preacher from Texas, was the prominent leader there.

Numerous new Protestant denominations began to form as Pentecostalism spread, beginning with the Assemblies of God, the Pentecostal Church of God, the International Church of the Foursquare Gospel, and the Open Bible Standard Churches. Still other young but established denominations such as the Church of God and the Church of God in Christ took on Pentecostal beliefs.

What nearly all Pentecostal denominations shared was a conviction that Christian experience was incomplete without the sanctifying and empowering work of the Holy Spirit and that the "baptism of the Holy Spirit" is validated by the evidence of "speaking in tongues," as well as by additional signs, including prophecy, visions, exorcism, and divine healing.

As early as 1914 several within Pentecostalism began to proclaim "Oneness," or "Jesus Only," a somewhat modal view of the Trinity that allows for different manifestations of God but suggests that there is ultimately only one divine person.

Oneness Pentecostalism typically insists upon rebaptism in the name of the Lord Jesus Christ alone rather than the Father, Son, and Holy Spirit. Its relationship to other Pentecostal denominations and to traditional Christian bodies concerned with theological orthodoxy remains ambiguous and controversial at best, depending in part upon how its theological claims are understood.

The Pentecostal movement was preceded by widespread, overlapping teachings among 19th-century evangelical Protestants about the need for a victorious "Higher Life" made possible by the filling of the Holy Spirit, about the importance of Holy Spirit crisis sanctification to purify the believer from sin, and about Jesus Christ as Savior, Sanctifier, Healer, and Coming King, thereby intertwining Christological and pneumatological emphases.

Pentecostalism strongly affirms all four of the latter themes as basic to the Christian life—Christian conversion, the Pentecostal work of the Holy Spirit who purifies and empowers the Christian believer for service, divine healing, and the imminent return of Jesus Christ in power and great glory as a motive for holy living and missionary endeavors.

Religious demographers now recognize a third type of Pentecostalism called the Neo-Charismatic movement. It actually consists of two or more rather distinct elements. One is the so-called Third Wave of evangelicals, who wholeheartedly affirm supernatural signs yet who diligently attempt to avoid the ecclesiastical schisms, upheavals, and controversies that frequently accompanied the first two waves. A much broader, more amorphous form of the Neo-Charismatic movement numerically dwarfs every other type of Pentecostalism.

It consists of the many thousands of independent Christian groups and denominations that have sprung up across the modern world more or less independently from traditional Roman Catholic, Orthodox, or Protestant influences.

Their founders often claim direct revelation from God by means of dreams or visions. Although these indigenous church bodies may prove difficult to classify, they are generally far closer to Pentecostal beliefs and practices than they are to other Christian traditions.