Baptisms in the Southern Baptist Convention have fallen to a 20-year low, a trend that is setting off alarm bells in America's largest evangelical denomination.

The number of people baptised in Southern Baptist churches and ceremonies, an important indicator of conversions and denominational growth, fell in 2007 for the third year in a row by 5 per cent to 345,941.

That was the lowest number since 1987, a trend on the minds of many of the 7,000 delegates known as "messengers" attending the SBC's annual meeting in Indianapolis.

This year's theme is called "Fulfilling the Mission" and the logo pointedly depicts a picture of a baptism in progress.

For Southern Baptists, a decline in baptisms is a worry because a major tenet of their faith is to spread it. Many believe the "unchurched" are doomed to an eternity in hell.

"We should always be concerned when baptisms dip. It's about salvation...We are commanded to go and preach the Gospel to every person," said Tommy French, a 77-year-old pastor from Baton Rouge, Louisiana.

Several delegates interviewed by Reuters expressed their concern in such terms: fewer baptisms meant fewer saved souls. For Southern Baptists, a public baptism in water is a key rite of the conversion experience.

The trend of falling baptisms also has broader cultural and political implications as the 16-million member SBC is a big part of the Republican Party's conservative Christian base.

Twenty-five per cent of US adults now count themselves as "born-again" or evangelical Christians, making the movement one of the fastest growing and most influential in America. A slowdown in its growth could have a ripple effect on politics and other areas of American life.

Several Southern Baptists interviewed took it as an unhealthy sign of "weakness" or misguided attempts to find accommodation with the broader secular culture that some regard as corrupting and even satanic.

"We are using corporate-style marketing and worship services. It's a performance orientation that lacks authenticity," said JD Perry, also from Baton Rouge.

For SBC evangelist Jim McNiel of St Louis, the drop in baptism numbers was a sign that the biblical "end times" described in the Book of Revelation were drawing near.

"I see two factions. You have one for believers but you also have a faction from Satan and there is a strong battle looming," he said.

Total SBC church membership also dipped slightly last year to just under 16.3 million but "primary" or regular worship attendance increased slightly to 6.15 million.

Falling baptism numbers have also been attributed to the cultural background of new converts, who include a growing number of Hispanics who are seen as reluctant to take "the dunk" because it is considered a final step that breaks most ties with their past.