(RNS) One hundred years after the exuberant worship ofPentecostalism started a wave of revival and the creation of newdenominations in this country, members of this newest major branch ofChristianity are celebrating the enormous growth of their religiousmovement.

The anniversary is being marked by denominational gatherings, theupcoming Pentecostal World Conference and a new 400-page bookchronicling the movement now estimated to include more than 500 millionworldwide.

"Beginning with only a handful of people in 1901, the number ofPentecostals increased steadily to become the largest family ofProtestants in the world by the beginning of the 21st century," writesthe Rev. Vinson Synan, author of "The Century of the Holy Spirit: 100Years of Pentecostal and Charismatic Renewal."

But as their movement has grown, Pentecostals have become harder toidentify. They retain their long-standing commitment to evangelism andvibrant worship services, but speaking in unknown tongues -- one of themost dramatic and signature elements of their worship -- is heard lessfrequently.

"The Pentecostals are getting more mainline and some of them aretrying to be respectable," Synan said in a recent interview. "And then alot of people come in who love the worship and never speak with tonguesand they join the churches. You have maybe half or less of Pentecostalsspeak the tongues. The rest of them believe in it, but they just don'tspeak it."

Sherry DuPree, the immediate past president of the Society forPentecostal Studies, said she is personally saddened by the trend. Now,more churches are known by the physical signs posted outside theirbuildings that declare their ties to Pentecostalism rather than thespiritual signs that used to emanate inside the church doors.

"The sign of the Spirit in some of them is gone," said DuPree, amember of the Church of God in Christ. "I hate to say so."

The Rev. Thomas Trask, general superintendent of the Assemblies ofGod and chairman of the Pentecostal World Conference, said one of theemphases of the gathering May 29-31 in Los Angeles will be onrecapturing the focus on the Holy Spirit that long has been prized byPentecostal Christians.

"There's still the work of the Spirit that makes it happen," hesaid. "It isn't organization. It isn't plans. It's God that has to doit."

He referred to a favorite Scripture from the Old Testament book ofZechariah: "`Not by might, nor by power but by my spirit, saith the Lordof hosts."'

Thousands of leaders of Pentecostal and charismatic churches fromabout 80 nations are expected to attend the triennial conference, whichmeets in this country for the second time since it began in 1947.

The vast array of representatives is part of a movement that beganearly in 1901 in Topeka, Kan., when a woman was baptized in the HolySpirit and began speaking in tongues at a small Bible college. Themovement gained global fame five years later, with the Azusa Streetrevival in Los Angeles.

Although the Los Angeles revival was unusual for its inclusion ofpeople of both races, the Pentecostal movement has struggled with racerelations since that time, with many denominations forming along raciallines. In 1994, during the so-called "Memphis Miracle" black and whitePentecostal leaders came together for reconciliation and formed theinterracial Pentecostal/Charismatic Churches of North America.

Synan, a former general secretary of the Pentecostal HolinessChurch, said some of the largest Pentecostal churches in the country areintegrated but overall racial reconciliation in the movement remains "ahuge challenge."

There also is a theological divide between Trinitarian Pentecostalsand Oneness Pentecostals. The latter, who comprise about 10 percent ofPentecostals in the country but are not recognized by some Trinitarians,believe that Jesus was the only person in the Godhead and baptize in"Jesus' name" rather than "in the name of the Father, Son and HolyGhost."

The movement has been known for its growth among diversedenominations and ethnic groups and its advancement of women. TheAssemblies of God, the world's largest Pentecostal denomination, has atotal of 15,000 ordained ministers in the United States and 4,000 -- or27 percent -- are women.

"We just believe that that is right and there's a place for women inministry within this church," said Trask, whose denomination held itsfirst Women in Ministry conference in March.

In the 1960s, the movement among established Pentecostaldenominations spread to mainline Protestant churches and to Catholiccongregations, where its adherents are called charismatics.

In addition to tongues, some charismatic and Pentecostal worshippersexperience other spiritual gifts, such as prophecy or healing. Whilesome Christians believe such manifestations of the Holy Spirit ended inbiblical times, Pentecostals and charismatics view them as modern-dayphenomena that continue to aid in evangelism.