"Dear Loyal," Reagan began. "I hope you'll forgive me for this, but I've been wanting to write you ever since we talked on the phone. I'm aware of the strain you are under and believe with all my heart there is help for that. . . . "

The letter dated Aug. 7 is not part of the presidential records publicly available at the Reagan Library. I came across it earlier this year, in a cardboard box of Nancy Reagan's personal effects. The library gave me access to them as part of my research on a biography of the late first lady.

I quote the letter here with permission of the Ronald Reagan Presidential Foundation and Institute, which has also allowed The Washington Post to reproduce it.

The discovery of this intimate missive, four pages of White House stationery randomly tucked in a file, stopped me. You do not have to be a believer yourself -- or believe that Reagan's policies were perfectly aligned with Christian teachings -- to appreciate what this private letter said about him.

I could sense Reagan's earnest intensity, how carefully he had collected his thoughts. Not a word of his small, round script was crossed out. Had he written and revised several versions, sending the one that said just what he wanted it to? Near the end were three watery smudges. Spilled coffee? Someone's later tears?

His language did not have the speechwriter-polished sheen we associate with the president who came to be known as the Great Communicator. It was an intimate, humble profession of faith. He was "Ronnie," assuring his father-in-law: "We've been promised this is only a part of life and that a greater glory awaits us."

It was "a miracle," Reagan wrote, that "a young man of 30 yrs. without credentials as a scholar or priest" had "more impact on the world than all the teachers, scientists, emperors, generals and admirals who ever lived, all put together." [...]

The letter to his father-in-law -- the only man who would ever come close to Ronnie in Nancy's estimation -- revealed how marital fidelity intertwined with Reagan's religious beliefs. He saw it not only as a source of happiness in this life, but a reward in the next.

Loyal Davis and Nancy's mother, Edith, who themselves experienced early divorces, were in many ways a model for the Reagan marriage.

"Loyal, you and Edith have known a great love -- more than many have been permitted to know. That love will not end with the end of this life," Reagan wrote. " . . . all that is required is that you believe and tell God you put yourself in his hands."

Did the letter have any impact? Nancy Reagan, who was with Loyal Davis when he died, and who saved the letter he received from his son-in-law, would later claim that her father did turn to God at the end of his life.