Prayers have been said for Norman Kember at his local church after a new video of the Iraq hostage emerged.
The video is the first news of the men for seven weeks
Congregation members at north London's Harrow Baptist Church spoke of their relief at seeing him alive on the tape.
Reverend Bob Gardiner warned that Mr Kember would have some stories to tell from Iraq that would be "uncomfortable" for the British government.
Peace activist Mr Kember, 74, was seized in Baghdad with two Canadians and an American on 26 November.
The video, dated 21 January and aired by al-Jazeera TV on Saturday, says four hostages will be killed if the kidnappers' demand that Iraqi prisoners are freed is not met.
The Foreign Office is assessing the tape.
Mr Kember's fellow captives are Canadians James Loney, 41, and Harmeet Singh Sooden, 32, and American Tom Fox, 54.
The group were travelling with Canadian-based international peace group Christian Peacemaker Teams when they were seized by a group calling itself the Swords of Truth.
Speaking during Sunday's service, Mr Gardiner said Mr Kember, who has worshipped at the church for 40 years, had gone to Iraq because he worried his faith was too comfortable.
Mr Kember, of Pinner, had "always been one to be challenging traditional ways of looking at things and doing things", he said.
TIMELINE OF ABDUCTION
26 November: Norman Kember abducted along with three other peace workers
30 November: Video footage of the hostages aired on al-Jazeera
2 December: Second video of the hostages aired. Their captors threaten to kill them unless their demands are met by 8 December
8 December: Third video is aired on al-Jazeera showing Mr Kember and US hostage Tom Fox wearing orange jumpsuits, blindfolded and shackled. The captors' deadline is extended to 10 December
28 January: New video of hostages, dated 21 January, aired on al-Jazeera
"He would always come with a radical voice in our church meetings and our committee meetings.
"I guess he will have stories with new insights about the situation in Iraq and we will listen.
"They may be uncomfortable stories, they may be uncomfortable for us, they are certainly likely to be uncomfortable for the government."
The video is the first news of the men for seven weeks.
One friend at the service said Mr Kember, who edited the church magazine, was "a lovely man".
"We're just glad it [the tape] shows he is still alive."
Al-Jazeera said it had aired the whole of the latest recording but the Qatar-based TV station would not say how it had obtained the tape.
The grainy 55-second film offers the first evidence the four are still alive since the release of a previous video in early December.
The latest tape includes a message from the kidnappers repeating their demand that all Iraqi prisoners be freed.
The kidnappers had originally set a deadline of 8 December for this demand to be met. This was later extended by two days to 10 December and there had been no news of the hostages since.
BBC correspondent Nicholas Witchell, in Baghdad, said the group was making political demands which could not be met.
He said the US had released 420 prisoners, including five women, two days ago but there were many thousands of people still in detention.