An award-winning American scientist noted for his landmark discoveries of disease genes, and leadership of the Human Genome Project to map the entire human DNA, has described his journey from atheism to Christian belief to an international audience in England.

"There really is no conflict between faith and reason," Professor Francis Collins told the CS Lewis Foundation's international summer institute, Oxbridge 2008, on at St Aldate's Church, Cambridge, on Wednesday.

"As a committed materialist in college, I assumed the physical was all there was," said Collins, who in 1977 at the age of 27 completed a career change from chemistry to medicine and became a doctor. This, he said, forced him to confront pain and death face-to-face. "That was a dramatic turn for me. The concepts were not hypothetical anymore."

Through encounters with patients, pastors and, finally, by reading "Mere Christianity" by CS Lewis, Collins realised, "I had never really looked at the evidence. Atheism had only been a convenient pathway. I had to decide what was really the truth but I thought that faith and reason were on opposite poles."

"Mere Christianity" began life as a series of lectures given by Lewis in 1943, and the best-selling book that followed had a profound effect on Collins. "Even in the first few pages, all my arguments about faith just fell apart. It was breathtaking ... Lewis remains my best teacher," he said. Within a year, Collins had become a Christian.

Before a packed audience in Cambridge, Collins cited evidence for his beliefs based on the moral law and mathematical and universal laws. Defending his position as a "theistic evolutionist", Collins said that his beliefs as a Christian and his research as a scientist had led him to the view that faith and reason are compatible.

Collins, who has been involved in identifying the genes that cause cystic fibrosis and Huntington's disease, formally retired on Friday, stepping down as director of the Maryland-based National Genome Research Institute.

Taking as his theme "The Language of God: A Scientist-Believer Looks at the Human Genome", Collins told his audience about his DNA work in mapping the 13 trillion gene pairs of the human organism. The DNA, Collins explained, is the "instruction book of the cell" and is made up of a double strand of chemical information coded as single letters. He added that having mapped the 3.1 billion letters, the genome project had made the information accessible to the worldwide scientific community as unpatented knowledge available for benevolent uses, most especially preventative medicine and gene therapy.

On 3 November 2007, Collins received the Presidential Medal of Freedom, his nation's highest civil award, for his revolutionary contributions to genetic research.

Francis Collins' lecture in Cambridge was one of a number of plenary presentations that make up Oxbridge 2008, which concluded on Friday. The summer institute moves to Cambridge until next Thursday under the theme of "Imago Dei? The Self and the Search for Meaning".