Every U.S. president has been a member of a church, except for one: Abraham Lincoln.

Famously opaque on the subject of religion, Lincoln’s personal faith was something even his closest friends said they couldn’t figure out. Though he became more interested in religious questions toward the end of his life, “Honest Abe” never directly identified himself as a Christian—even after he realized it could hurt him politically.

“He once spoke of how not having any kind of noticeable religious profile had levied what he called a tax on his popularity with the voters,” says Allen Guelzo, a professor of Civil War-era studies at Gettysburg College and author of Abraham Lincoln: Redeemer President. “It was something that he was aware of, something he tried to cope with, and yet he wouldn’t go the distance of trying to pretend that he was something that he wasn’t.”

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After Lincoln was assassinated in 1865, Mary Todd Lincoln defended her late husband from questions about his faith while also giving conflicting accounts of what he actually believed, or when he started to believe it. Despite his later assertion that it was God’s will to end slavery, some of his close friends admitted that they had no idea what Lincoln’s broader religious views were because he was so secretive about them.

Over 150 years later, it’s even harder for us to tell what Lincoln believed. Still, his Bibles are among the most famous of any president’s. Both Barack Obama and Donald Trump used the so-called “Lincoln Bible” from his inauguration at their own swearing-in ceremonies. Ironically, Lincoln didn’t actually acquire that Bible until he showed up for his first inauguration in the middle of the night. A Supreme Court clerk brought him a Bible to swear on, and that’s the one most associated with him today.