At Three Crosses Church Pastor Ken Walters urges his parishioners to join him in song and scripture. The charismatic 58-year-old extends his arms skyward and belts out melodies praising God.
While the small Assemblies of God congregation goes through all the traditional trappings of a Pentecostal service, there is one notable absence: speaking in tongues, a defining trait of the faith.
The 40-member church near Los Angeles is among many nationwide that are reducing or cutting out speaking in tongues as they become more popular and move to the mainstream. It's a shift that has unsettled some more traditional Pentecostals who say the practice is at the heart of a movement that evolved from an interracial revival and remains a spontaneous way for the poor and dispossessed to have a direct line to God.
They question the wisdom of placing less emphasis on a tenet that has defined Pentecostalism for more than a century.
"It's different now," Walters said. "People don't like to stand out if they don't have to."
As the religion becomes more widely accepted, Walters said, there has been a tendency for large Pentecostal churches to downplay the differences between Pentecostalism and other well-known Christian denominations.
The Assemblies of God, one of the nation's largest Pentecostal denominations with 3 million members in the U.S., has 66 million members worldwide. Assemblies officials worried about the decline in messages in tongues - or spirit baptism - at a general council meeting last month.
"This is a long-developing phenomenon," said Harvey Cox, an expert in Pentecostalism and professor of religion at the Harvard Divinity School. "They don't want what appears to be objectionable to stick out or be viewed with suspicion."
Meanwhile, newer strands of Pentecostalism - often with roots in other countries such as Nigeria and El Salvador - continue to emphasize the practice in church as well as in personal prayer, Cox said.
While all Pentecostals accept speaking in tongues as a "gift of the Holy Spirit," these smaller, niche congregations aren't afraid to embrace the practice and don't care whether it scares some away, he said.
Pentecostalism represents one of the fastest-growing segments of global Christianity. At least a quarter of the world's 2 billion Christians are members of the Pentecostal faith or related charismatic movements, according to the Pew Research Center.
For the first decade, the movement was mainly composed of poor white and African-American worshippers. Influenced by the spiritual renewal of the Azusa Street Revival - a Pentecostal revival meeting that took place in Los Angeles in 1906 - the Assemblies grew with interracial services that included speaking in tongues, prophecy and faith healing.
Occasionally, parishioners were "slain in the Spirit," falling to the floor following an encounter with the Holy Spirit.
Pentecostals believe speaking in tongues might be an unlearned human language- as the Bible claims happened on the Day of Pentecost - or it might be the language of angels. Studies show that words spoken when delivering messages in tongues lack the components and patterns of a true language.
At his service in a small chapel in the West Valley Christian Center, Walters steps aside after reading scripture and introduces a guest: Nick Farone, a pastor who runs a Christian center in Louisiana. Farone uses his time on stage to preach returning to the basics of the faith. Parishioners in the pews nod their heads in agreement, swaying back and forth.
After the service, Farone placed his right hand on his forehead and began to speak again. This time, the words were impossible to understand, a long, rambling string of sound. He had just spoken in tongues, he said later.
"This is our power," he added, acknowledging he was unsure of what he had just said. "We shouldn't be ashamed."