Tim Keller has asked for prayer after being diagnosed with pancreatic cancer. 

The bestelling author and theologian shared the news on Twitter, writing: "Less than 3 weeks ago I didn't know I had cancer. Today I'm headed to the National Cancer Institute at the NIH for additional testing before beginning chemotherapy for pancreatic cancer next week back in New York City."

It is not the first time that Keller has had cancer.   The founder of Redeemer Presbyterian Church in New York City survived thyroid cancer in 2002.

Following the new diagnosis, he said he was trusting in God and feeling good in spite of the news. 

"I feel great and have no symptoms. It was what doctors call an 'incidental pickup,' otherwise known as providential intervention," he said.

"I have terrific human doctors, but most importantly I have the Great Physician himself caring for me."

He added, "Though we have had times of shock and fear, God has been remarkably present with me through all the many tests, biopsies, and surgery of the past few weeks."

Keller is asking Christians to pray that God would use medical means "or his direct intervention" to make the cancer "regress to the point of vanishing". 

He also asked Christians to pray that the side effects of treatment would not interfere with his writing and speaking, that his family would be "comforted and encouraged", and that he and his wife Kathy would use the cancer as an "opportunity to be weaned from the joys of this world and to desire God's presence above all".

In another post on Twitter, he referred to Hebrews 12:1-2 as he spoke of his desire to run the race set before him with joy "because Jesus ran an infinitely harder race, with joy, for me."