The Archbishop of Canterbury is among the Christians expressing their sorrow over the killing of hostage David Haines at the hands of Islamic State militants.
The 44-year-old aid worker's beheading was shown in a video released on Saturday night.
It has been strongly condemned by British Prime Minister David Cameron, who has vowed that Britain will take "whatever steps are necessary" to keep the country safe and bring the killers to justice.
Archbishop Justin Welby used his Twitter account to ask every church in the country to pray for Haines's family, saying he had been "evilly killed in the place he was serving in love for its suffering people".
In comments to the BBC later on Sunday, the Archbishop described the aid worker's murder as "an act of absolute evil, unqualified, without any light in it at all".
He said there was a sense that in places where militants have taken hold "the darkness is deepening".
"It's being done in the name of faith, but we've heard already today faith leaders from Islam across the world condemning this," he continued.
"What's going on is a power-seeking activity. Faith is often used as a hook on which to hang other desires, and this is a desire for power and influence, and faith is being twisted to enable it to be used to gain power and influence for their own unspeakably evil ends.
"So today there is that sick sense of horror at the wickedness we see, a deep sense of compassion for the family, and prayer that they may be comforted by the presence and light of Christ in a very, very dark time indeed."
The Archbishop made further comments in a sermon at Bristol Cathedral in which he reminded people that despite such darkness in the world, Christ is not remote.
"And so where is Christ in that? On Holy Cross Day we are reminded above all that he is with David Haines, that he is in the depths of evil and the depths of our own suffering because of the cross," he said.
Alpha founder and vicar of Holy Trinity Brompton, Nicky Gumbel, tweeted: "Heartbroken by evil murder of compassionate and courageous aid worker who was in Middle East to ease the suffering of others. #David Haines."
Executive director of the Conservative Christian Fellowship, Colin Bloom, tweeted: "Thoughts & prayers with family of #DavidHaines - beheaded by evil men acting with impunity. #ISIS #ISIL #BokoHaram #AlShabaab - all evil."
A Rocha UK's Ruth Valerio tweeted: "Sickened by the news of David Haines this morning. Simply can't think of anything meaningful to say."
TV presenter Diane Louise Jordan shared: "Rip DavidHaines Forgive them for surely they can't know the evil they've done. My heart goes out to your family dear David."
Tearfund's Katie Harrison said she hoped the loss of Haines would remind COBRA - the British government's emergency council - of the need to "plan any intervention without jeopardising aid efforts".
"Lots of lessons from Afghanistan," she said.
