The Late Show comedian told Rev James Martin on the Faith in Focus program that he stopped being a Catholic in his 20s.
"I had lost my faith in God, to my own great grief," Colbert said.
"I was sort of convinced that I had been wrong all this time, that I had been taught something that wasn't true."
Colbert explained that a stranger gave him a pocket Bible when he was 22-years-old.
After reading Jesus' words from the Sermon on the Mount, he was "absolutely, immediately lightened".
"I stood on the street corner in the cold and read the sermon. And my life has never been the same," he added.
When asked about how he views God today, Colbert replied: "It's Jesus Christ. Not an old man with a beard.
"If I can love, I can be free. When I think of love, I think of God, and when I think of God, I think of love."
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