The latest London killings are clear proof that the church desperately needs to promote the Christian understanding of what it means to be human.

The tragic deaths of Jack Merritt and Saskia Jones at a prisoner rehabilitation event a week ago is a terrible reminder of just how twisted human beings can become when they adopt, or have been captured by murderous ideologies.

I witnessed this kind of violence first hand during the brutal war that resulted from the break up of the former Yugoslavia. I met those whose families had been massacred by their neighbours simply because of their ethnic identity, and I have stood outside a church which had been reduced to ruins because it was where "the other side" worshipped. Is it any wonder, then, that former Archbishop George Carey could suggest that man is "God's problem child"?

Our fallen human nature can reveal itself in rampant populism, too, which is why, like so many others, I have been deeply upset to see parts of the press and some politicians making political capital out of this latest terrorist incident.

And Jack Merritt's father David summed it up well when he said that his son would be "seething" at how his death was being used to perpetuate an agenda of hate.

"Don't use my son's death, and his and his colleague's photos - to promote your vile propaganda," he wrote on Twitter. "Jack stood against everything you stand for - hatred, division, ignorance."

Now, David Merritt might not be a Christian but his challenge is consistent with what Jesus has taught us. If we want to live God's way, in other words if we want to live the right way, we must do all we can to love our enemies, not hate them. And we should pray for them, not persecute them.

In the same way, Christians can take inspiration from Jack Merritt's determined attempt to deradicalise and rehabilitate those who have been blinded by warped ideologies.

After all, Jesus operated on the basis that people could change, and His optimism has been vindicated time and time again over the centuries. Murderers, thieves and reprobates of all kinds have undergone complete transformation. As Christians we have the added assurance that when we set out to reach people, we do so in the power and the presence of the Holy Spirit. 

But Jack Merritt's terrible death is also a sobering reminder that it is very easy for people to give the impression that they have changed when, in reality, it's simply a cloak to disguise what they are really thinking. The Bible says there needs to be repentance, a word that points to a new mindset or a new way of seeing things.

William Wilberforce understood this when he decided to campaign against the practice of slavery. He clearly understood that it would not be enough to pass new laws and that it needed a change of heart, not just a change of legislation.

Human behaviour can be enforced of course, or at least it can be to some extent. But ideas and beliefs are in a completely different league altogether. I have learned that as a pastor. I can do all I can to explain and to persuade people to see things God's way but in the end I have to recognise that they will ultimately decide what they choose to believe.

Jesus understood this, too, which is why the apostle John could say of those who seemed to be His disciples "There are those of you who do not believe," for Jesus had known from the very beginning "which of them did not believe and who would betray Him".

Sadly, given the complexity of human nature, we can't operate on the basis that human beings are inherently good. We are inherently sinful, too, which is why it is sensible to recall the pessimistic words of the prophet Jeremiah: "The human heart is the most deceitful of all things, and desperately wicked. Who really knows how bad it is?"

And so while we live in hope and work for transformation and rehabilitation, we must be constantly on our guard. Jesus sends us out as sheep among wolves, which means we have to be "as shrewd as snakes and harmless as doves". 

Rob James is a Baptist minister, writer and church and media consultant to the Evangelical Alliance Wales. He is the author of Little Thoughts About a Big God.