President Donald Trump and first lady Melania Trump greet former President Barack Obama and Michelle Obama at the state funeral for former President George H.W. Bush at the National Cathedral. (Chip Somodevilla/Getty Images)

Forget the sold-out arena tour, the best-selling hardcover of 2018 and the celebrity endorsements: The moment when Michelle Obama crossed over from being a famous first lady to just plain famous happened in a pew at Washington National Cathedral. Let me explain.

Following protocol at the state funeral for former president George H.W. Bush last week, Michelle sat with her husband, the Clintons, the Carters and, awkwardly, the Trumps. After President Trump shook hands with his predecessor, Barack Obama, Michelle, dressed in a smart black pantsuit, leaned over and offered what appeared to be a curt “Good morning.” (It’s worth noting that Hillary Clinton, who Trump has said should be jailed, didn’t even look in his direction.)

The slightly raised eyebrows, the borderline smirk and a sharp look that spoke volumes — the shade! It was an instant Michelle Moment, a viral snippet from the somber occasion that seemed to solidify what so many say they love about Mrs. Obama: her authenticity.

“She delivered her true self in the role that she has today,” says Elaine Swann, author of the etiquette guidebook “Let Crazy Be Crazy.” Michelle’s behavior, Swann says, was a skillful example of greeting someone unpleasant with grace but not warmth: “She was just cordial enough.”

It’s as if the world finally realized just how boxed in Michelle was for eight years. “As a former first lady she obviously has a lot more space to really express herself,” continues Swann. We know from her memoir, “Becoming,” that Michelle will “never forgive” Trump for questioning her husband’s citizenship as the frontman of the birther movement, which, she wrote, was “deliberately meant to stir up the wingnuts and kooks.” Now, the lioness is loose — and Michelle’s reaction to Trump during that awkward pew exchange was a perfect example of her public evolution.

A similar viral moment happened in 2017 when Michelle and Barack welcomed the Trumps to the White House on Inauguration Day. A camera caught her confusion as she tried to find a place for a surprise Tiffany box gifted from the incoming first lady. Later, Michelle’s face offered a window into her innermost thoughts at the swearing-in ceremony. She didn’t look happy. She didn’t look like the hostess in chief she had been for two administrations. “I stopped even trying to smile,” she wrote in “Becoming.”

About that book: The former first lady has spent the past month selling millions of books and crisscrossing around the country (and abroad) like a woman on a mission. A mission to what exactly? To take over our Instagram feeds? The world? Perhaps simply to take back her own narrative.

She is no longer “the wife of” and has stepped out onto her own stage. Notice the power suits in place of the conservative sheaths. Consider the s-bomb dropped in an arena of 19,000 the previous week (when she denounced Sheryl Sandberg’s “lean in” philosophy). And that face at the funeral? All of it can be seen as an announcement of sorts: that she’s done with the pomp and circumstance.

This realness is what fuels her popularity. It’s the raw candor that so many believe is missing from most politicians and celebrities. “What drew us to her,” says Swann, “was her honesty and her authenticity, and I think we’re just seeing another layer of that at this point in time.”

Myra Gutin, a Rider University professor and author of “The President’s Partner: The First Lady in the Twentieth Century,” describes both Michelle and Hillary Clinton as “rock stars,” women who turned post-White House life into enduring power. But unlike Clinton, whose fame is still qualified by politics, “Michelle Obama crossed that line between first lady and celebrity.”

Gutin’s list of most influential first ladies includes Betty Ford for her work highlighting breast cancer and addiction (“She rose to some pretty high heights”) and, of course, Eleanor Roosevelt (“She is our gold star even now”). But Michelle is in a class of her own because she’s no longer defined by the first lady role. She colors way outside those lines. “She’s probably the most popular Democrat in the country,” says Gutin.

“When they go low, we go high” are arguably Michelle’s most famous words. But her post-White House stardom is far removed from campaign rhetoric and instead rooted in relatability. So what happens when a former first lady stops being polite and starts getting real? Fans love her even more.

Helena Andrews-Dyer is co-author of The Post’s Reliable Source column.

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