BOSTON — What was the best moment yesterday in an afternoon full of them?

What was the best moment in a Garden lovefest?

What was the signature moment in an afternoon full of them?

Maybe that came with 3:22 left in the third quarter, the Celtics already up 34, the game long since decided, when Kevin Garnett dived for a loose ball near midcourt, his body sprawling across the parquet. The ball squirted away from him and was rolling toward the baseline at the Celtics’ end of the court when Ray Allen dived on the floor for it, followed by Paul Pierce also diving on the floor for it as it rolled out of bounds.

If you needed any more obvious example that this was a game the Celtics were simply not going to lose, maybe that was it.

The Big Three all diving on the court on the same play in a game that long had been decided.

The Big Three showing that Celtic Pride is more than just some old bumper sticker, a remnant from the glory days.

The Big Three showing that there was no way in this world that the Hawks were going to come in here yesterday and steal this game.

Forget the fact that this game was essentially over by the end of the first quarter when the Celtics were up 11 and it was becoming obvious that the fourth game of this series here in the Garden was going to be very similar to the first three.

No surprise there, really.

The surprise was that there was a Game Seven in the first place.

No matter.

The Celtics did what they had to do yesterday, taking control of this game right from the beginning and sending a message to the young Hawks that there would be no fairy tales happening in this building, not yesterday anyway. They sent an early message that they were going on in these playoffs and the Hawks were going home, end of discussion.

It was an important message, for truth be told, the Celtics had a certain crisis of confidence following the loss Friday night in Atlanta, They knew they weren’t supposed to have lost three games in Atlanta, knew they were not supposed to be in a Game Seven yesterday with the Hawks, complete with the spectre of ignominy hovering over them if they were to have lost. They knew that they had a lot to lose yesterday.

“We gave away a couple of games in Atlanta,” Pierce said, “and we were going to take care of business today.”

Pierce also said that this was the first trip through the playoffs for this team, and it’s easy to forget that. Pierce, Garnett, and Allen are veterans, certainly, have played in a lot of big games through the years, but never together. This is all unchartered territory, the playoffs, a Game Seven, the expectations, all of it.

“It was a good test for us to go Game Seven,” he said.

He was sitting in the interview room afterwards alongside Garnett. It’s been their custom all season. They come out together. They often echo the other’s statements, as if there is a Celtics view of the universe and it’s their job to state it. So Garnett added that this series with the Hawks was important, for “we learned a lot about ourselves.”

What came across was that this is still a work in progress. Forget the 66 wins. Forget having the best record in the NBA in the regular season. They both know that that’s already yesterday’s news, know that what happens in the playoffs is how this team is going to be judged, and that the standards are high as all those championship banners that stare down from the rafters.

So Garnett said that Pierce and coach Doc Rivers had said in the locker room immediately after the game was that this team learned a lot about itself in this series with the Hawks.

And the most important thing it learned?

That the playoffs is all about paying attention to detail. Playing every possession. Always bringing energy. To play with a sense of urgency, Trusting each other.

“You know, a lot of the players have been through a lot of playoff games,” Rivers said, “but we haven’t been through anything as a team.”

Rivers also said that until this series started the Celtics hadn’t played a meaningful game in a month and a half, and he’s right about that. In a sense, the regular-season almost came too easy for them, getting off to a quick start, essentially going through the entire year without a whole lot of pressure on them.

That changed Friday night in Atlanta.

Losing for the third time on the road to the Hawks, a team they were supposed to have little trouble with.

Coming back here yesterday for a Game Seven, everything on the line.

“We stuck together through adversity,” he said, “and adversity reveals who you are,”

For he knew that the Celtics needed a big effort early yesterday, not only to set the tone, but also to take the Hawks out of the game, the message that nothing had changed since Game Five, that the Celtics were at home and the Hawks had no chance, end of discussion.

“I just told them that I thought we played terrific basketball tonight,” he said. “I told them that that was the Celtics.”

Yes, it was.

And seeing Garnett, Pierce, and Allen all dive headlong on the floor for a loose ball when they were up 34 in a game that already had been decided was maybe all you had to know, the best moment in an afternoon full of them.

breynold@projo.com