The Celtics told one another they were going to win. And they said it so often and so emphatically that at some point it transformed from a hope to a belief to a truth.

OAKLAND, Calif — At some point early in the Celtics’ game against the unstoppable Warriors, a mantra developed. It was recited during timeouts and breaks and any time there was a quiet moment in the never-quiet Oracle Arena.

“We knew,” Avery Bradley said. “Ask Isaiah [Thomas]. He’ll tell you. I mean, we literally went in the huddle like, ‘We’re gonna win this game no matter what,’ from the beginning of the game.”


It is one thing to have this confidence on a normal night in a normal game against a normal team. But this was different.

There, on the other bench, were the Warriors, who are chasing the best regular-season record in league history. There were the Warriors, who had won an NBA-record 54 games in a row on their court.

It didn’t matter. Celtics coach Brad Stevens would not let doubt creep in.

“He kept saying, ‘Look, this is ours,’” Thomas said.

And when Stephen Curry’s 3-pointer rimmed out in the final seconds, and Harrison Barnes’s last-ditch attempt was off, too, an improbable final score glowed on the scoreboard for the stunned fans here to see.

Boston 109, Golden State 106.

It was the Warriors’ first home loss since Jan. 27, 2015, and it was one of the most stunning results of this NBA season. For the Celtics, the win confirmed what was already becoming clear as this season evolved.

“We know we can compete,” Thomas said. “And we know we’re a team that guys don’t really want to play when it comes to the playoffs.”

Amid the rigors of an 82-game NBA schedule, in which good wins and bad losses ultimately blend together, players usually do not let one game become more significant than the others.


But this one resonated. In Boston’s locker room afterward, music blared and players shimmied.

“That’s a hard thing to do, to come in and beat a team like the Warriors, especially on the streak that they were on, in their own house,” guard Marcus Smart said. “We knew it was gonna take everything we had.”

Watch: Jared Sullinger’s fourth-quarter basket in paint

Sully with the big-time second effort! pic.twitter.com/Lj10dfCWer — Boston Celtics (@celtics) April 2, 2016

The last time these teams met, at TD Garden in December, the Warriors were 23-0 and the Celtics nearly silenced that streak, too. They scraped and clawed before falling in double overtime. But the Warriors were without All-Star Klay Thompson and Barnes that night. It was unclear if the Celtics could stick with them when they were whole.

Then on Friday morning, Celtics forward Jae Crowder was ruled out with lingering soreness from an ankle sprain that had sidelined him for 8 games. But this team has crafted some of its most pristine moments when least expected.

“We told each other, ‘We’re not here for vacation,’” Bradley said. “That’s what we kept saying the entire time before the game. We were here to win this game, not just come out here and say we played hard.”

The Celtics led 97-88 with 5:30 left, but Golden State answered, because it always answers. A 3-pointer by Thompson and an acrobatic layup by Curry pulled the Warriors within 103-101 with 1:29 remaining.

With the Celtics leading 105-103, Amir Johnson slipped and lost the ball and the Warriors jumped on it and called timeout. But on the next play, Johnson stole the ball right back.


The Warriors fouled Evan Turner, who made two free throws with 18.1 seconds left to give Boston a four-point lead. Then Barnes drained a 3 from the left corner to pull Golden State within 107-106 with 10.2 seconds to play.

After a timeout, Smart floated a daring inbounds pass from the sideline toward Thomas, who caught it before the Warriors could and converted a tough layup, making it 109-106 with 8.3 seconds left.

At the other end, Curry pump-faked once and found himself open for a deep 3-pointer near the top of the key. It is a shot he has made so many times, and in this building there seemed to be a belief he would make this one, too. But the ball rimmed out, and Barnes’ desperation 3 at the buzzer never had a chance.

“Every one of them, I think, is going down,” Curry said. “All things considered, it was a pretty good look.”

After a scoreless first half, Thomas sizzled in the second and led the Celtics with 22 points. Jared Sullinger had 20 points and 12 rebounds, and Turner added 21 points and 3 steals. It would seem Boston would need to be near-perfect to pull off this upset, but it missed 11 of its 30 foul shots and still prevailed.

The Warriors lost despite converting 20 of 43 3-pointers, the most ever made against the Celtics. Curry led Golden State with 29 points, but he will probably remember this game most for the one shot he missed. He had a season-high 9 turnovers, and the Warriors had 22 as a team.


“We just wanted to make sure they felt us on the defensive end,” Smart said. “We’ve got guys that can guard out here, so we wanted to make sure we built the pressure up.”

Watch: Marcus Smart’s fourth-quarter 3-pointer

Marcus Smart beats the clock on a step-back 3! pic.twitter.com/JWwbp3xPpG — Boston Celtics (@celtics) April 2, 2016

The Celtics took a 45-43 halftime lead, and the crowd at Oracle Arena that is known for being so raucous seemed almost desolate.

Really, though, it was tough to tell which team had been most fortunate. The Celtics led despite Crowder’s absence, despite missing six free throws, and despite getting no points from Thomas, Kelly Olynyk or Smart. The Warriors, meanwhile, trailed by just two points despite committing 13 turnovers and getting just 11 total points from Thompson and Curry. The only certainty was that the second half would juicier than the first.

When Curry drilled two 3-pointers during the first minute of the third quarter, it looked like one of Golden State’s patented bursts was imminent. But Thomas, an All-Star himself, scored 13 points over the first five minutes to give the Celtics a 60-58 lead.

He had 18 points in the third quarter and Curry scored 21, as the Celtics took an 82-79 lead to the fourth. Slowly, the impossible began to appear possible for Boston. With 8:41 left, Smart—who had missed 18 consecutive 3-pointers over his last six games—drilled one from the top of the key to give the Celtics their largest lead, 93-83.


The Warriors made a final push, but in the end, so did the Celtics. Stevens has consistently said he wants to see progress as this season unfolds, and although there was an obvious indicator of one significant step taken on Friday, he said it was not the fact that his team had won.

“The progress,” he said, “was in the poise.”

Watch: Jonas Jerebko’s dunk of inbound pass

Jonas Jerebko with the slam off the inbound pass! GSW didn't see that one coming! pic.twitter.com/S4OMWV7R8G — Boston Celtics (@celtics) April 2, 2016

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.