Tim Keller, who is battling pancreatic cancer, has opened up with HTB pastor Nicky Gumbel about how the diagnosis has changed him and wife Kathy.

"The Meaning of Marriage" author was diagnosed with pancreatic cancer in June and has since undergone chemotherapy. 

At the time of his diagnosis, Keller said his doctor was upfront in telling him "there's virtually no cure for this". 

"It's a very difficult cancer to treat," the pastor said. 

He said the diagnosis led to a "bit of a role reversal" because he had always been the "strong" one taking care of his wife Kathy, who has been sick with Crohn's Disease for many years. 

"Now God has just decided well, we're going to reverse the roles here and you're both going to have to get used to what it means to trust me in this new role," he said. 

In terms of his prospects, he said "we don't know".

While statistically, most people with pancreatic cancer die within a year of their diagnosis, Keller said his cancer had been caught early and that the prospect of "keeping it at bay" for a longer period of time looked "pretty good". 

"So it's not likely right now that I would be dying within a year," he said. 

When Gumbel asked whether he was dealing with any fear, Keller admitted that he and his wife "cry just about every day" about him having cancer but said his fear wasn't about death or disease but about Kathy having to face life without him. 

"My fear isn't dying.  My fear is actually leaving her behind and that's her biggest fear as well. It's a terrible fear," he said.

While Kathy "has her own sorrows" about the prospect of outliving her husband, he said they were clinging to the belief that if this happens, it's because "there's things that God has for her to do".

Reflecting on the positive changes in his faith life, he said that both he and Kathy "never want to go back spiritually to where we were before the cancer diagnosis". 

"We never want to go back to that because in spite of all the things I've already preached, I wasn't a hypocrite exactly but the reality is that most of us say we need to depend on God but we actually think we've got it sorted," he explained. 

"We feel like we've got everything under control because we've thought this out, we've got savings, we've got these people [in our life]." 

He said that while people "know technically" that they are mortal and that God is in charge, they don't "experientially" believe it "until life gets beyond your ability to control it". 

When that happens, that's when people can turn to God and know that "He really is there" and He is "enough", the pastor said. 

"I'm actually happier than I've ever been on a given day," he continued.

"I enjoy the things around me in a way that I've never enjoyed them before - I see them as gifts of God - and I enjoy my prayer life more than I ever have in my life."