WALTHAM — I have come to see Al Horford as the shibboleth.

In the religion of basketball, he is the test that determines entry into the Hall of People Who Know What The Hell’s Going On.

I have listened to and read the running commentary on the Celtic forward/center/guy-who-guards-just-about-anyone. I have heard the believers and the detractors, and I’ve spoken to countless people who make their living in this game.

And after all of it, I just don’t understand why this is an issue. With all due respect, those who believe Al Horford is not an elite player or worth his max contract just … don’t … get … it. Forget about the belief at this keyboard, to hold either of these positions puts you on the opposing side of the brightest people in the NBA. The Celtics are in the Eastern Conference finals with a 1-0 lead, and Horford’s fingerprints are all over it — even the ability of the younger players to perform this well.

In terms of the four-year, $113 million contract Horford signed here in 2016, it wasn’t as if the Celtics were bidding against themselves. Among those lining up to max him out were Houston and Washington.

Most basketball people see beyond the numbers when it comes to Horford, and we’re not just talking about the testimonials that come when any coach is asked about an opposing player. (The most tepid accolade you’ll get is that Player X is “very dangerous.”) No, coaches bring up Horford unprompted and even in private conversations.

But the numbers on his individual stat sheet will never match those on his contract, and that will be enough fuel for his detractors.

Those same people would, of course, take the player who averaged 30.1 points on 54 percent shooting for his career over the guy who averaged 15.1 points on 44 percent from the floor.

And they would have thrown aside Bill Russell for Wilt Chamberlain.

No one’s putting Al Horford into their category, but the point is that there’s more to this than points — and rebounds. If Horford gets five rebounds on a given night, but consistently blocks his man off the boards and allows his teammates to get to the ball, he is more valuable than someone who regularly leaves his man to get 10 rebounds while giving up several offensive caroms and second-chance points to the opposition in the process.

I’ve stated with conviction for years that there wasn’t a day in Bill Russell’s career when Wilt Chamberlain wasn’t a more individually talented player in terms of size, strength and skill level. If the two engaged in a basketball decathlon, Wilt was winning.

Ah, but Bill understood the object of the game better. He understood the bigger picture and how he could affect the planetary movements. Ergo, Bill Russell is the greatest winner in the history of team sports.

(And now that we seem to be caught up in this Michael or LeBron argument for The Greatest Player Ever thing, let me just state that any such discussion that doesn’t at least include Russell and Wilt — and Magic Johnson — is flawed on its face.)

As strong as the fully healthy Celtic roster may be, if the club didn’t have an Al Horford, they’d be moving mountains to try to get one. Versatile big men who know how to fit are rare. In fact, even while Kyrie Irving and Gordon Hayward are likely “better” players, Horford may be the most indispensable.

“I believe so,” Marcus Smart said yesterday. “It’s hard to replace him. He brings so much. And with Kyrie and Gordon out, he’s stepped up his game even more. And with this young group, he’s a very important leader for us.

“Al is a unique player. He’s one of those guys that’s really changed the game with how a big plays. He’s able to shoot the 3, and he’s quick enough and has the ballhandling skills to beat a big off the dribble. And if you put a smaller, quicker guy on him, he’ll use his strength and take him into the post and beat him there. It’s really hard to match up with him.”

So what are Horford’s detractors missing?

“The everyday consistency,” said Brad Stevens. “He comes to work the same way with the same mindset, with the same focus, every day. When he speaks, everything he says is worth listening to, and when he plays, he makes everyone around him better.”

Which sometimes can make Horford look worse. If leaks spring elsewhere and he has to cover for them, his man could have a big night. But that’s never stopped Horford from doing what’s needed.

“He is a totally selfless player,” said assistant coach Jay Larranaga. “He is a winner. I would hope people realize that, based on his lifetime winning percentage from high school to college to the NBA. The guy is just a big-time winner. Everyone else on the court plays better when he’s out there.”

Atlanta hadn’t made the playoffs for eight years when Horford arrived from two national championships and a 92-19 record in three years at Florida. The Hawks signed Dwight Howard when Al left. They traded Howard away after one year and 43 wins. This season, their 24 wins were tied for third worst in the NBA and they fired their coach.

So leave aside the monetary comparisons. Al Horford isn’t Kevin Durant or LeBron James, but his value to the Celtics should be evident by now — especially in light of how hard it is to fill his sneakers.

Put it this way, do you want to play Cleveland tonight without him?