The Baptist Union of Great Britain (BUGB) chose London as the venue for this year's General Assembly in tribute of the first British Baptist church, founded by Thomas Helwys in the Spitalfields area in 1612.
The assembly opened last night with a look back over four centuries of Baptist achievements in Britain and the world, from the pioneering mission work of William Carey, to the legacy of preacher Charles Spurgeon and civil rights leader Martin Luther King.
A message from the Queen read to the audience congratulated Baptists on this "auspicious occasion".
The assembly is meeting under the banner of "Beyond 400". General Secretary of the BUGB Jonathan Edwards said: "We're here because we want to capture God's vision of where we go next."
New BUGB President Chris Duffett was sworn in during the opening session.
In an address to the assembly, he said that the calling on Baptists was to show how "big-hearted" God is through their lives.
"It's great to celebrate 400 years but what of the next 400 years? What hope have we got as brothers and sisters?" he said
"Our God is a great big-hearted God [and] he wants to do more and more through us."
Duffett, an ordained evangelist, is touring the nation with his message as part of the Big Hearted Tour, where he said the emphasis was more on "doing" evangelism than training in evangelism.
"How are people going to see our big-hearted God? It's through you and me," he said.
In his address, Duffett said it was time to do church differently. He said that churches in Britain needed to be more vibrant and more missional.
"We need an inside out church," he said.
"Many Baptist churches I visit are similar to sex shops and bookies. You can't really look in – the frosted glass and high windows.
"People need to be able to see and experience us out and about."
Tonight's address will be given by American Baptist and popular speaker, Tony Campolo.