The Archbishop of Canterbury has told ITV News International Affairs Editor Rageh Omaar that Islamic State (IS) represents an "extraordinary, once in a millennium threat".
Justin Welby pointed out the dire threat to Christians in the Middle East from the extremist group.
He recalled a meeting of regional Christian leaders in which one had called Islamic State "the worst threat we've faced since the Mongol invasion of 1259".
International Affairs Editor Rageh Omaar reports
Archbishop Welby also praised the "extraordinary courage" of many Islamic leaders who have spoken out against Islamic State, but called on religious leaders to continue to voice their opposition to the group.
He rejected the suggestion that British Muslim leaders could have been more bold in coming out against IS.
"The ones I know have been extremely bold and I can't say how much I admire their courage, because it's much harder for them than it is for me," he said.
As for his own solution, Archbishop Welby said the answer lay in people of different faiths offering a brighter alternative to the IS' bloodthirsty rhetoric.