Charles Schulz was widely applauded for a long list of achievements. The creator of the Peanuts comic strip was a Pulitzer Prize nominee, and his comics earned him an Emmy, Peabody, and Congressional Gold Medal. Sixteen years after his death in 2000, Schulz is still the third top-earning deceased celebrity, trailing only Elvis Presley and Michael Jackson. He even changed the way Americans talk, inserting phrases like “Good grief!” and “security blanket” into the national vocabulary.

But Schulz also revolutionized his industry by using his strip to subtly raise religious questions about the Bible, prayer, the nature of God, and the end of the world. Schulz was a devoted Christian; unshell the Peanuts and you’ll find the fingerprints of his faith. By mixing Snoopy with spirituality, he made his readers laugh while inviting them into a depth of conversation uncommon to the funny pages.

“Many familiar with the Peanuts strip don’t think of Charles Schulz as a Christian pioneer,” said Stephen Lind, the author of A Charlie Brown Religion: Exploring the Spiritual Life and Work of Charles M. Schulz. “But he was a leader in American media when it comes to both the strength and frequency of religious references.”

More than 560 of Schulz’s nearly 17,800 Peanuts newspaper strips contain a religious, spiritual, or theological reference. To put this into perspective, Schulz only produced 61 strips featuring the famous scene where Lucy pulls the football away from Charlie Brown as he tries to kick it. Particularly later in his career, the religious references came so frequently that pastors and religious publications regularly requested permission to reprint Peanuts strips, which Schulz almost always granted.

Schulz’s most recognizable reference to religion occurs in the Charlie Brown holiday special exploring the “true meaning of Christmas.” Realizing that the holiday’s secular accouterments did not form the essence of Christmas, Linus reads the story of Jesus’s birth directly from King James Version’s account in the Gospel of Luke. At the time, less than 9 percent of Christmas episodes and specials contained religious references.

On rare occasions, Schulz stepped out onto more spiritually shaky ground. In 1963, when debates over the role of religion in public school were raging, Schulz penned a somewhat controversial strip in which Sally recites the pledge of allegiance from her classroom desk and concludes with a resounding, “Amen!”

“I preach in these cartoons, and I reserve the same rights to say what I want to say as the minister in the pulpit,” Schulz once said.

But the Peanuts preacher was not of the “hellfire and damnation” variety you might expect to find in a fundamentalist church. He did not attempt to shame skeptics, level judgments, and stated he “didn’t have any axes to grind.” Schulz was also no evangelist and made no effort to convert non-believers. (He actually once offered a critique of evangelists with a strip in which Linus hands out religious tracts to neighborhood families.) Instead the cartoonist just let who he was and what he believed leak into his strips.

“You can grind out daily gags, but I’m not interested in simply doing gags,” Schulz would say. “I’m interested in doing a strip that says something and makes some comment on the important things in life.”

Schulz converted to Christianity shortly after returning from a deployment in World War II, and the experience sparked a love inside of him for sacred literature. He became a voracious reader of theological commentaries, and the margins of his personal Bible were filled with hand-written notes. He was a long-time Sunday School teacher at churches in the Midwest and California, even leading one group through a study of the entire Old Testament.

This may be why many of the religious references in the Peanuts were drawn directly from sacred texts. In June of 1952, the somewhat sad and self-deprecating Charlie Brown borrowed Solomon’s words from Ecclesiastes 1:14: “All is vanity!” In December of 1955, a shivering Snoopy found solace in Jesus’s words from John 16:33: “Be of good cheer, Snoopy … Yes, be of good cheer.”

Sometimes the Bible references were clearly cited. When he catches Snoopy taking food out of the refrigerator, Charlie Brown pulls out a Bible and quotes from the Ten Commandments: “Look, it says here in Exodus, ‘Thou shall not steal.’” Snoopy borrows his book, flips the page and hands it back. “Deuteronomy 25:4 …” Charlie Brown reads, “Thou shall not muzzle the ox while he treads out the grain.”

But often, they were more cryptic. When Linus asks Snoopy, “Does it bother you that the Bible doesn’t speak very highly of dogs?” the beagle replies with a reference to one of Jesus’s teachings, “Sure it bothers me, but I just turn the other muzzle.” In a famous strip from 1959, Linus built a sandcastle that the rain washed away. Linus concludes, “There’s a lesson to be learned here, but I don’t know what it is …” But many readers would have recognized the allusion to Jesus’s parable about a man who built his house on sand in Matthew 7, and Schulz later said that this was exactly what he intended.

Schulz’s willingness to include his faith in his work cut against the grain of his era. Chic Young, creator of the Blondie comic strip, reportedly warned mid-20th century illustrators to avoid mentioning religion in their work. Lind says that almost no other mainstream cartoonist dared to include religious references in their strips at the time.

As Schulz said in 1966, “Just the mere fact of quoting from the Bible, of course, for a long while was forbidden in comic strips because somehow they just didn’t want you to go near these areas.” But, according to Lind, the cartoonist’s bravery “opened the flood gates” and other cartoonists soon picked up their pen to share spiritual thoughts as well.

Schulz was criticized by some devout believers for making religious references in “such a lowly thing as a comic strip.” But a comic strip may have been the perfect place for Schulz raise spiritual questions. “When readers come to the end of the panel, there is a gap not only between two rectangles, but also the action contained in each and the reader must then fill in what happened, creating a sense of mental ‘closure’ so that the episode makes sense,” Lind writes. “As the reader fills in this narrative leap, they begin to connect with the scene, for they helped create it.”

This was exaggerated in the Peanuts strips, which tended to have an excessive amount of white space throughout and sometimes very little action. Additionally, the “pint-sized” characters were able to raise adult questions in a disarming way.  The result is that readers become participants of the strip’s conversation instead of merely spectators.

When he had Linus ask, “Do you ever pray, Lucy?” Schulz was nudging readers to reflect on their own prayer habits. (Schulz penned 40 different strips that addressed prayer.) When Charlie Brown confessed to Lucy, “Sometimes I wonder if God is pleased with me,” Schulz was tapping into a common stirring among many to live a “good life” that even a perfect God would deem laudable.

“The more you read a certain set of ideas, the more likely you are to think about them,” said Lind, who cites a study indicating that children may mimic the types of words they read in comics.

Given the success of Peanuts—at its peak, read by 355 million in 21 languages across 75 countries—quantifying the true impact of Schulz’s religious content is difficult. How many conversations around water coolers or dinner tables during the last half of the 20th century were shaped by Schulz’s strips? How many were taped to cubicles or refrigerators as spiritual encouragements?

Later in his life, Schulz’s began to refer to himself as a “secular humanist,” as his theology became less traditional. This did not mean he was no longer a Christian, but rather that he now believed other faiths might also provide legitimate paths to God. He was also less certain about other Christian doctrines, such as the existence of a literal heaven.

The more Schulz wrestled with faith, the more he led his readers to do the same. In a 1985 strip, Sally asked Charlie Brown, “When we die, will we go to Heaven?” Charlie responded, “I like to think so.” This mixture of hope and skepticism reflects Schulz’s own evolving faith, which came to assume an increasingly central role in his work. When you map religious references in Peanuts over time, the trend line shoots up and to the right.

Schulz’s final strip was scheduled to run on Sunday, February 13, 2000. It contained a sketch of Snoopy typing the cartoonist’s farewell letter to his readers. As newspaper printers across America prepared his strip for delivery, the cancer-stricken Schulz slipped into bed and passed away. His wife reported that he died peacefully.

“Little things we say and do in Christ’s name are like pebbles thrown into water,” Schulz once said. “The ripples spread out in circles, and influence people we may know only slightly and sometimes not at all.”

For half a century, he threw pebbles. Some were larger and more swiftly lobbed than others, but most of them found their mark. They tumbled into homes and hearts, urging readers to consider life’s deeper questions. These rocks were delivered by uncommon messengers—boys with blankets and beagles with typewriters. Sometimes they made readers chuckle and other times they induced an “aaugh!” But Schulz always aimed to challenge readers’ minds, and on occasion, touch their souls.