REP. JOHN LEWIS, as an activist, speaks not just about “the movement,” but also in the language and sense and unshakeable cadence of movement itself. It’s the diction and conviction of moving forward; this civil-rights icon doesn’t believe in going backward. He does believe, however, in going back.

Today, more than a half-century after the historic Selma march — that day Lewis thought he was going to die — the Georgia Democrat is back in Alabama, the state of his birth. The congressman appeared Saturday in Birmingham, 53 years after four little girls were killed there in the 16th Street Baptist Church bombing. And he appeared Sunday in Selma, getting together with friends and supporters 51 years after hundreds of nonviolent protesters led by Lewis were beaten and teargassed there, horrifying the country and sparking the 1965 passage of the Voting Rights Act.

“You go back to be renewed and to be inspired and to be uplifted,” says Lewis, who also returned to Selma last year, marching alongside President Obama at anniversary events.

The 76-year-old congressman relives that history not only in person but also on the page, with his graphic memoir that recounts how he first became a nonviolent protester as a teenager and endured pain and terror while leading the Student Nonviolent Coordinating Committee (SNCC). He promoted the series’s second installment last summer with a mock march with schoolchildren at Comic-Con, and this summer he’ll release the end of the trilogy, “March: Book Three.” It begins with the 1963 church bombing, works its way through protests in the Deep South and concludes at the White House, with the signing of the Voting Rights Act.

“March: Book Three” is illustrated by Nate Powell and co-authored by Lewis staffer Andrew Aydin, and its cover captures the Selma showdown at the Edmund Pettus Bridge — a concept that Powell says required a five-month collaborative process. In the image, you can see Lewis leading the marchers, as Sheriff Jim Clark tells his armed officers to advance and Alabama State Trooper John Cloud leads his men. The authorities use not just batons but also cattle prods — Aydin calls them “professional racists.”

“It shows the apex of the movement,” Lewis tells The Post’s Comic Riffs. “It was only on that bridge, in a sense, that we got the Voting Rights Act of 1965, which freed and liberated so many people, whether black or white, whether straight or gay.”

But before Selma, Birmingham had to endure tragedy in 1963, which is where the book begins, and Lewis helped stage subsequent protests. “Book Three” then moves to the 1964 Mississippi Summer Project — the voter-registration effort that endured attacks by the KKK, police and state and local authorities. In June 1964, three young registration workers were abducted and killed. “It sent shock waves not just through the civil-rights workers,” says Lewis, “but also through the [new] volunteers.”

During the six weeks that the three workers’ bodies were missing, Lewis and other SNCC members were among a small group that “went out one evening to try to locate them,” Lewis says. “When I think back, it was very dangerous.”

“To be candid, you don’t believe this happened in America,” Lewis says of the murders of James Chaney, Andrew Goodman and Michael Schwerner. “These people did not lose their lives in Vietnam or the Mideast or South America. They were killed right here in our own country.”

In “March,” Lewis also includes his chance meeting and conversation with Malcolm X in a Kenyan cafe, several months before Malcolm was assassinated in February 1965.

Such tragic events set the stage for the Selma showdown. When he reads his own book, even reliving this history can be an ordeal for Lewis: “It becomes so painful, I have to put it down for a moment.”

“March’s” tone turns triumphant five months later, though, when President Johnson hosted Lewis and other civil-rights leaders in the Oval Office. “He told us he was going to sign the act,” Lewis recounts. “After that morning, he wanted us to go back and get people registered.”

The congressman still smiles at the memory of Aug. 6, 1965. “It was a day of celebration. There was an elderly man who was almost 100 and he got registered. He said: ‘For the first time, I can go home and be free.’ ”

Lewis hopes “March: Book Three” will inspire “another generation to continue to push and pull and to speak up and to speak out.” Yet the congressman says he also sees threats to everything he has worked for — he’s worried that the Supreme Court, when it finally gains a replacement for Justice Scalia, could roll back elements of the Voting Rights Act. And he is concerned that some leaders could undermine messages of nonviolence and unity.

“People don’t want to go backward, but [Donald] Trump as president would be a major step back,” Lewis says. “It would be something we haven’t witnessed in a very long time in our country. … You’re dividing people by race and class and nationality. That’s not America. To insult people — it’s not presidential. It’s not what America is all about.”

Last Thursday, before and after a House vote, Lewis rode an elevator with his congressional colleagues. “I catch myself saying: ‘Have a good weekend, sister. Take care, brother.’ It doesn’t matter whether they are Democrats and Republicans. We have to continue to reach out to people … and not divide them,” he says.

“Humanity, civility and moderation are needed, but it can be hard to turn it back on [in America] when many turn it off.”