"Don't be afraid!" said the archbishop of Canterbury to the packed cathedral.
The congregation stretching out below his pulpit was led by Denmark's stately queen, Margrethe II, and a dozen clergy who included not only Lutherans but Catholic and Eastern Orthodox priests, South Africa's Desmond Tutu and two Buddhist nuns.
But it was the children at the head of the processional signaling the start of this Ecumenical Celebration for Creation -- held midway through the two-week climate summit in Copenhagen -- who had set the tone.
As the organ boomed and voices soared with "All creatures of our God and King/Lift up your voice and with us sing ..." up the aisle, two by two, the children led the adults.
In their hands they held the reasons for fear -- strange offerings for any church service.
First, held out in the cupped hands of the first three pairs of children, were pieces of dead bleached coral from the Pacific, "a symbol of rising sea temperatures, polluted, suffering and dying ocean worlds," as the program informed the congregants.
Then came three pairs of African children carrying cobs of dried-up African maize -- "symbol of drought and desertification, of failed crops, human hunger and suffering."
Then came children, each carrying stones uncovered by retreating glaciers in Greenland, "symbol of melting polar worlds, of rising sea and river levels, and loss of life-giving mountain water resources."
Recent estimates from scientists and international agencies say sea level could rise 4 feet within the next 90 years, that the oceans are 30 percent more acidic than at the start of the industrial age, and that some 300,000 people are already dying every year -- all because of human-induced global warming.
These are frightening statistics for a diverse global population, facing the idea that it has a collective ability to alter basic life-support systems of the earth itself.
So, for the theme of his sermon, the archbishop had chosen fear.
It recalled other appeals to psychological self-awareness in the face of enormous daunting tasks:
Franklin D. Roosevelt's "The only thing we have to fear is fear itself" as he took over the presidency during the Great Depression;
Pope John Paul II's initial and repeated "Be not afraid!" to crowds in St. Peter's and across communist Poland as he helped launch a nonviolent campaign that would eventually see the peaceful dismantling of the Soviet Union.
"Love casts out fear," preached the archbishop. "We must learn to trust one another in this world of limited resources" in order to do "what we must do to save creation itself."
The question of collective courage is at the heart of many issues debated by negotiators from 192 countries assembled for the climate summit here.
Their purpose is to agree on how humanity will, with coordinated economies and actions, do nothing less than, in effect, rewire the planet's energy system, rendering it within only a few decades almost completely free of carbon emissions.
The necessary changes to local economies and governance are frightening if for no other reason than just the magnitude of the change and the speed with which experts say humanity must achieve it all.