“I feel we can still make a run at this thing.”

— Mickey Callaway, July 7

The most famous line in the history of the Mets was hatched on the afternoon of Monday, July 9, 1973. It began when Tug McGraw, suffering through his most difficult season, had lunch with an old friend of Gil Hodges’ named Joe Badamo, an insurance salesman on Long Island who also dabbled as a motivational speaker.

The Mets were languishing in last place, McGraw’s ERA was sitting at a grotesque 6.20, he’d blown five of his 16 save opportunities, his record was a ghastly 0-4. Badamo offered McGraw a simple message that took hold in a small corner of his brain: “No matter what,” Badamo told him, “You’ve got to believe in yourself.”

Later that day, as the Mets gathered in the Shea Stadium clubhouse before a game with the Astros, the team’s chairman of the board, M. Donald Grant, showed up with a few corporate guests fresh off a three-martini lunch to show off his baseball toy. Grant was not yet the infamous grim reaper he would become a few years later, but the players had already marked him for what he was: “We called him ‘Stuffed Suit,’ ” one said years later, “though never to his face.”

Grant addressed the players to put on a show for his guests, and he began to ramble badly, and the gist of his talk was this: “If we didn’t believe in you as players then you wouldn’t be a part of the New York Mets.””

McGraw, hiding a smirk behind his glove, heard his magic word.

And lost his mind.

“YOU GOTTA BELIEVE!” he screamed, stopping the speech cold. “YOU GOTTA BELIEVE, MR. GRANT! YOU GOTTA BELIEVE, EVERYONE!”

He turned to one of his stunned teammates.

“Do YOU believe? YOU GOTTA BELIEVE!!!!”

Grant walked out, furious. McGraw’s teammates urged him to go to Grant’s office and explain himself. Reluctantly, he did. He found a red-faced executive, enraged at being shown up in front of his men’s-grille brethren. He meekly apologized. Grant waved him off.

“The only thing that will keep you here,” Grant fumed, according to the excellent book “Swinging ’73” by Matthew Silverman, “is if we start winning some ballgames.”

And, well, you might have heard. Soon enough, they would.

McGraw’s mantra is every bit a part of the Mets’ permanent fabric as any other element of the team’s history, and not far behind is the observation Yogi Berra, the manager, would make a few weeks later: “It ain’t over till it’s over.” There are, in truth, few sports teams that have such a well-known slogan attached to them and, for better or worse, the Mets have co-opted all things “believe” across the decades.

Which brings us to Mickey Callaway, who for most of this season has served as an unwitting salesman for the franchise’s primary asset: belief, so much of it usually at odds with logic. Of course, almost every time he said something to that effect, it became a punch line because … well, we are a lot more jaded in 2019 than we were in 1973. What was once considered hopeful is now huckster.

But he kept selling it nonetheless. Endlessly. Tirelessly.

June 11: “I’ve been with a few teams, whether it is the .500 mark or getting above four games above .500, there are always these marks that seem to be difficult and then once you eclipse them, you can finally take off.”

June 17: “I mean, we’ve got a lot of talent. The talent is definitely here. A lot of guys who’ve played in the playoffs, a lot of guys who’ve won before, it’s just about all coming together at the same time and getting on a little winning streak.”

June 25: “It will all even out. We will be better.”

July 3: “I think every game for us the rest of the season is going to be huge because of where we’ve put ourselves.”

And, well … he wasn’t lying. And it turns out, he wasn’t wrong. This doesn’t mean Callaway has become Casey Stengel all of a sudden, of course. But, then, nobody in 1973 was confusing Yogi for the Ol’ Perfesser, either. This newspaper ran a poll, asking if he should be fired (Yogi won in a landslide, not surprisingly; it’s unlikely Callaway would’ve fared as well if we’d printed a similar ballot in, say, June).

(Asked about his manager a day before he tried his hand at being Knute Rockne, Grant said, “We would not consider changing managers unless forced to by public opinion,” which really, REALLY makes you wish that Twitter had existed during the reign of M. Donald Grant.)

Interestingly, Berra’s main hassle in 1973 was the exact same one that nearly ended Callaway in 2019: a bullpen that betrayed him game after game, led by McGraw, whose first four months in ’73 were every bit as catastrophic as Edwin Diaz’s in 2019. And in much the same way Callaway has steadfastly tried to keep Diaz’s spirits up, Berra did the same, and more, for Tug.

“I’ll die with him if I have to,” Berra said on the same day as Grant’s speech.

Callaway has made no such oaths about Diaz yet, but about his ballclub at large he’s been steadfast: All along, he believed — that word again — they’d find a way to get what they have now: three games, at home, in front of what will surely be an amped-up Citi Field, against the Nationals. If they are truly to declare themselves, now would be a fine time to do it.

Maybe Diaz will never roam the clubhouse screaming, “TIENES QUE CREER!” to his teammates and to Jeff Wilpon, should he stop by pregame. Still: McGraw finished ’73 with a 1.64 ERA and a .167 batting average against across his final 25 appearances, he went 14-for-14 in save opportunities with five wins in seven decisions.

If Diaz channels that element of Tug?

Then maybe there really is a reason to believe.