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On Jan. 6, the Boston Celtics blew a 25-point lead to the New York Knicks, and lost in heartbreaking fashion when RJ Barrett banked in a 3-pointer at the buzzer. That was the Celtics' fifth loss in seven games, and dropped them to 11th place in the Eastern Conference at 18-21. 

A frustrated and disappointed Ime Udoka sat at the podium and let his team have it after that loss. The Celtics' first-year head coach called them out for a "lack of mental toughness to fight through those adverse times."

"I feel like he's 100 percent right, to be honest," Robert Williams III said at the time. "We get rattled a lot, especially when we're facing adversity. We've got to find in ourselves the fight to just come together."

Five months later, the team has not only come together, it has advanced to the NBA Finals for the first time since 2010. On Sunday night, the Celtics hung on by a thread down the stretch to beat the Miami Heat in Game 7 of the Eastern Conference finals, 100-96. 

The postgame message this time around was a bit different:

"Two Game 7s in the last two series," Udoka said. "Shows what I said about our group. That we fought through a lot of adversity this year. A resilient group. Tonight seemed to typify our season."

The Celtics still have their flaws. They're turnover prone, can be taken out of games at times by worrying too much about the officials, and have issues scoring in crunch time. They blew a 14-point lead to the Milwaukee Bucks in Game 5 of the second round, lost Game 6 of this series at home despite holding a late lead and saw their 13-point lead in the final few minutes of Game 7 get whittled down to two in the closing seconds. 

But even if they don't always make it easy on themselves, they always have a response. 

"That's what we do, we did all that on purpose to make it interesting," Jaylen Brown said. "No, I'm just kidding. But that's us. We've been responding all year, all season to adversity. Today was the biggest test, not just of the year but of our careers, to mentally come into a Game 7 away after losing on our home court, which was tough, and we got it done."

There were mini examples throughout this game. When their 17-point lead was cut to six at halftime (in part due to some questionable officiating), they went on a quick 7-1 run out of the break to push it back to double figures. When the Heat made another charge at the start of the fourth quarter to make it a three-point game, the Celtics rattled off an 8-0 run. And finally, when the Heat got it back down to two in the closing seconds, Marcus Smart hit two clutch free throws to help seal the deal. 

That was the mental toughness and resilience we've seen from this group throughout the playoffs. They're 3-0 in elimination games, including two wins on the road, have won two Game 7s and are still a perfect 6-0 following a loss. In fact, they haven't lost back-to-back games since late March. 

At times, it's hard to believe this is the same team that sat there on that fateful January night in New York wondering where everything had gone wrong. That defeat still lingers in Tatum's mind. He referenced it again on Sunday, calling it the "lowest moment" of the season. 

In another world, that could have been the beginning of the end for this group. Whether Tatum and Brown could play together was a constant topic of debate, not only in local Boston media, but on a national level. Marcus Smart's ability to run the team was in constant question, and he was subject to trade rumors yet again. There was skepticism, too, about whether Udoka was the right man for the job. 

Even internally, the doubts were starting to creep in. 

"It was tough," Tatum said. "Like truly. There were definitely some tough moments throughout the season where -- not doubt yourself but maybe question, right, question, can we do it? You start to realize how hard it is to win. You start to question yourself; are you good enough to be that guy?

"But I think you just trust in yourself, trust in the work that you put in to get to this point and continue to work. It can't rain forever. Good days were coming. I felt that we were -- whatever it was, one step away from clicking throughout the season, and obviously once we did, we haven't looked back."

Not when they blew that game to the Knicks and were sitting outside of play-in tournament position. Not when they choked away Game 5 to the Bucks in the second round and had to win two straight elimination games. Not when they fell apart down the stretch in Game 6 of the East finals and had to go back to Miami for a Game 7 on the road. 

No matter the situation, the Celtics were always confident in their abilities and eager for a chance to respond. For the past four months, they've been focused on the next game and the next opportunity. Now, they have their biggest one yet. 

"I think it's all right to enjoy this tonight and be happy because it's hard," Tatum said. "It's not easy -- clearly this is my first time being in the championship. It is not easy. We know we have a tough task ahead. They've been there many a times, they've won many a times. I'm looking forward to it."