Does war tend to strengthen or weaken the faith of those fighting? A man who has collected more than 80,000 soldier letters has an answer.
For Arthur Craig, a letter from his son, 27-year-old Staff Sergeant Brian Craig, is precious beyond words.
Brian wrote it from Afghanistan in 2002, just days before he would die in a blast with three other soldiers. The four were part of an explosives unit, trained to destroy rocket caches.
But this time, the enemy had booby-trapped the site.
Arthur Craig remembers his last phone conversation with his son. A sandstorm made it difficult to hear. But Arthur had an important question.
"Out of my spirit," Arthur said, "I asked him, 'Brian, how is your walk with the Lord?' and he started to tell me and we were cut off."
Twelve days after Brian's death, his parents found a letter from their son in this mailbox. Brian had written it to his dad right after that last phone conversation.
Arthur reads the letter, "You have asked me about my walk with Christ on the phone. It is strange that of all my experiences in life, here in Afghanistan I have really started to grow spiritually. Pray for me that I may be a good example of a man of Christ."
The words provide a glimpse of a soldier growing in his faith, even in the midst of constant danger. They would prove to be a great gift.
"It was a tremendous blessing," Arthur said. "That was one of the days that we were very down."
Andrew Carroll is the founder of The Legacy Project, a national effort to collect American soldier letters.
He included Brian's letter in his new book, Grace under Fire, which focuses on letters of faith.
Some are filled with hope, like this one from a teen Civil War soldier who lay dying from a gunshot wound: "I don't think I shall live to see morning, but my kind friends, I am a soldier of Christ. I will meet you all in heaven."
Other letters contain the great questions of life, like one from a World War I private: "How can there be fairness in one man being maimed for life, while I get out of it safe?"
Others recount dramatic spiritual growth.
This was from a soldier in World War II: "I always considered myself a good Christian until I was captured, and then I learned what a fool I had been and what it really means to have faith."
Despite the span of years, from the Revolutionary War to Iraq, Carroll says the emotions are still the same.
"Whether you go off with a musket or an F-16, you're confronting your own mortality," he said, "and this in itself engenders these very profound questions and answers."
Carroll began collecting soldier writings after a family fire in 1989 destroyed all of his personal correspondence. Several years later, he started a movement that has netted more than 80,000 letters.
Carroll calls them the "great, undiscovered literature of America," and they reveal, he says, a pattern of faith being strengthened in times of war.
"What really happens is when you're in a war zone," Carroll said, "your mind concentrates on what's truly meaningful and lasting in life. And what are those things? Faith and family. So it's not really a question that you're grasping onto something to help your own self-survival. It's that the mind and soul are clear and they see with greater insight and purity what's really meaningful in life."
Army Staff Sergeant Jeff Pugmire is the one man who survived the explosion that killed Brain Craig and three others.
Jeff had served with Brian for years, and wrestled to make sense of his death. One letter to his wife is also published in Grace under Fire.
"I can tell you, though, that we don't do it for money or fame or fortune," Jeff said. "Heaven knows I'll never be rich. We do it for one another, for the men and women that are next to us. But most of all, Jennie, I do it for you and our beautiful little girls."
Jeff believes that Brian and the others did not die in vain.
He said, "They gave their lives for something they believed in -- in something that was important to them. It says in John, 'No greater love has a man than this -- that he lays down his life for his friends.' That epitomizes Brian, Justin, and Jamie."
In the midst of their grief, Brian's parents decided to share their son's letter with others and it has yielded a rich harvest. Many have given their lives to Christ, or rededicated themselves.
Who would have thought that one letter, written on simple notebook paper, could produce so much for the kingdom of heaven?
Arthur reads, "Thank you for all that you do for me. Thank you for being a role model. Thank you for being not only parents, but great friends. I love you, Mom and Dad, so much. Love, Brian."