Today it is five years exactly since 16-year-old Jimmy Mizen was murdered in a bakery in south London. He would have been celebrating his 21st birthday.

It was an unprovoked attack – veins in Jimmy’s neck were severed after his killer, Jake Fahri, had thrown a Pyrex dish full of sausages at him in a rage. He is currently serving a life sentence for murder.

I interviewed Jimmy's parents for the next issue of Christianity magazine. Barry and Margaret Mizen have recounted the story of how their son died so many times, you might be forgiven for thinking they’d become immune to telling it. They have the air of people who are very used to being interviewed, but they’ve lost none of their humanity.

It’s always been this way for them. Even in the early days, people were asking who was doing their PR because of the gracious way in which they spoke to the press. The answer was no one. God, they say, was directing their speech as they talked to the gathered press about forgiving their son’s killer, and praying for his family.

"I can’t get them [Fahri’s parents] out of my mind," Margaret said at the time, "because what’s happened to Jimmy is the worst thing possible, but we’ve got such wonderful memories. They haven’t got wonderful memories for their son. All they can think about is the evil he’s done. My prayers are with the family, that’s all I can say. I don’t feel anger."

They are not alone. There are many Christian families who have spoken publicly about forgiving a terrible sin which has been committed against a loved one. But I am no less struck by how extraordinary the act is. How can we forgive someone who has committed the worst thing we can think of? If someone murdered my fiancé, or my mum or dad, could I forgive them?

Barry Mizen is quite pragmatic in his approach to forgiveness. It makes sense, so that you’re not weighted down with hate and fear; hating his killer doesn’t bring back your dead son.

But the Mizens also had another reason for forgiving Fahri: what God has done for them.

The forgiveness of God compels them to forgive the very worst kind of human sin. It doesn’t mean what he did doesn’t matter, and that he won’t have to live with the consequences. It doesn’t mean the Mizens won’t bear their painful scars of loss for the rest of their lives. But it does demonstrate the most incredible obedience to Christ, and opens up new possibilities for him to work in their lives. Perhaps that makes the call to forgiveness a bit easier. Forgiveness is about us getting closure and being released. It’s a law God’s given us because it’s good for our souls, not because he needs to be appeased.

That’s the theory. But when we have the privilege of meeting people such as the Mizens who have made this extraordinary promise of God a reality in their own lives, I feel we owe it to them to do likewise. We are a forgiven people. What arrogance would it be not to extend that gift?

Ruth Dickinson is the editor of Christianity magazine.