The Archbishop of Canterbury Dr Rowan Williams has paid tribute to the families of Abigail Witchalls and Anthony Walker in his Christmas sermon.
The tsunami was one of the most terrible disasters, said Dr Williams
At Canterbury Cathedral he praised the way they responded to horrific violence with Christian forgiveness.
He also told of the devastation of last year's tsunami, remembering it as "one of the most terrible disasters".
Anthony Walker was murdered with an ice pick in July and Abigail Witchalls was paralysed in a knife attack in April.
Mother of 18-year-old Anthony Gee Walker - a devout Christian - said she would follow Jesus' example and forgive his killers.
Anthony died in a racist attack at the hands of cousins Paul Taylor and Michael Barton in Huyton, Merseyside.
Both victims' parents have forgiven their attackers
In his address, Dr Williams said: "A few weeks ago, Gee Walker, mother of the murdered Liverpool teenager, Anthony Walker, told us that yes, she forgave her son's killers and yes, her heart was still broken.
"What made this so intensely moving was the fact that her forgiveness was drawn agonisingly out of her, without making her loss easier."
Abigail Witchalls, 26, was stabbed in the neck as she walked with her young son in the village of Little Bookham, Surrey.
Her mother, Professor Sheila Hollins, has written that she has forgiven Richard Cazaly - the man police say was behind the attack - and called his subsequent suicide the "real tragedy of the story".
"She wrote, not making light of her daughter's terrible ordeal or denying the complex evil of the action, but simply making space in her heart for someone else's fear and pain," Dr Williams said.
"Why remember what happened at Bethlehem, why resist the efforts to reduce it to a brief fling of sentimental goodwill in the middle of bad weather?
"Because of people like these. They have known in their flesh and nerves just what the difference is that Jesus makes; it is not comfort of easy answers, it is the sheer fact that miraculous love is possible."
Celebrating Christmas was important, he continued, because it marked the anniversary of the moment when human history changed.
"Because of the difference Jesus makes, a world in which the sanctity of life as an alien concept has given way to one in which the landscape has changed," he said.
"You may or may not believe what Christian doctrine says about the child in the manger but you will, consciously or not, be looking at the human world in a framework that Jesus Christ made possible."
He also warned of the need for vigilance against the loss of what Christianity brought.
"If we ever do come to forget not just the Christmas story but what it made possible ... the arrival of a different humanity, there is enough, sadly, in our idle and self-obsessed hearts to let the ancient world begin to creep back a little more."
Dr Williams opened his sermon by talking about the tsunami, nearly a year after it happened on Boxing Day 2004.
"For those most directly involved, the date of December 26, 2004, marks a brutal interruption - the death or injury of someone, terrible anxiety, bereavement, anger and bewilderment.
"But for all of us, the date will carry significance, for all of us something erupted into our comfortable consciousness. Like September 11 and, now, July 7 as well, it stands in the landscape or the map, a feature that will never be obliterated. That was when things changed," he told the congregation.