Addressing supporters on Facebook, he said it was an announcement he never expected to make and described the prognosis as "quite grim".
Despite this, he says he and his family are relying on God for healing.
"We are going to pursue healing aggressively, both medical and miraculous, relying on God and the fact that He is able to do immeasurably more than all we ask or imagine," he said.
"In the past few days my spirits have soared and sank as I pursue the Lord's will and consider what the future might look like, but never once have I doubted this: that Jesus is Lord, His blood has paid my ransom, and by His wounds I am healed."
The development means he's cancelling almost all speaking engagements and stepping back from his work with Ravi Zacharias International Ministries.
Qureshi does however plan to blog or vlog about his journey with cancer as an encouragement to others.
Calling on Christians to pray for him, he said: "Friends and family, may I ask you to fast and pray fervently for my healing? I do not profess to know the will of the Lord, but many of my close friends and confidants are convinced that this is a trial through which the Lord intends to bring me alive and refined.
"May His will be done, and may I invite you to seek Him in earnest, on your knees, fasting on my behalf, asking our Yahweh Rapha for healing in Jesus' name."
Nabel Qureshi's converted to Christianity from Islam after engaging in discussions with Christians while at university. He's subsequently written several books about the two religions. His title Seeking Allah, Finding Jesus was a New York Times bestseller.