He has been speaking days after terror attacks claimed by IS took the lives of 129 people in Paris.
Lord Williams told the 2015 Orwell Lecture at University College London that the media was wrong for 'dehumanising' Islamic State.
"Somehow the obstinate attempt to make sense of those who are determined to make no sense of me is one of the things that divides civilisation from barbarism, faith from emptiness. You have to try," he said.
He said he did not have "sentimental illusions that all you had to do was be nice to people" but added that you had to "imagine the other" and how they are feeling.
He added: "I think the hardest thing we face at the moment is how do we imagine the unimaginable mentality of somebody who thinks that God or justice or the future is honoured by slaughter and barbarity?
"If we gave up on it we would have given up on something colossal about our humanity."
Jeremiah Johnston is a professor in early Christianity at Houston Baptist University and author of Jesus and the Jihadis: Confronting the Rage of ISIS.
He told Premier: "I disagree, I think the Islamic State is the face of evil. It would be the same as saying a few years ago that we should humanise Hitler.
"I think we need to understand that we are living in world war three right now, whether we admit it or not, so humanising or not, it's doesn't escape the evil and the atrocities perpetrated by the Islamic State.
"We always want to humanise our fellow man, that's certainly a logical thing to think, it's a civilised thing to think, but the people that we are discussing are not civilised."
