The Post Sports Live crew previews the AFC and NFC championship games on Sunday in New England and Atlanta. (The Washington Post)

Aldon Smith stood in the San Francisco 49ers’ locker room last week and talked about relief, strength and identity. Others talked about being on the cusp of football history.

The face of one of the NFL’s most exciting teams is quarterback Colin Kaepernick, the electrifying, second-year budding superstar. But the 49ers’ soul is their defense, and eight days before the NFC championship game, that’s what Smith wanted you to understand.

It has brought the 49ers this close for the second consecutive year. But can it close the deal this time?

Said the second-year linebacker: “We’re a team that can rise to the occasion.”

When it has risen, it is because the defense is strong and complete. Defensive tackle Justin Smith missed the final two regular season games with an arm injury, and in the first of those games, the 49ers allowed 42 points to Seattle. He’s not healthy now, but he’s at least healthy enough to play. This is a relief to his teammates and to San Francisco fans, because when this defense is whole, it is one of the NFL’s best.

According to the statistics analysis Web site, the 49ers have the league’s second-best total defense, and their best skill is stopping the run. That forces teams, particularly those with traditional offenses, to pass more frequently, and that’s when defenders like Aldon Smith excel. He finished the regular season with 19.5 sacks, second in the leagueonly to Houston’s J.J. Watt. In November, Smith had 5.5 sacks against Chicago.

So it stands to reason why he and his teammates might’ve been eager to test themselves against the Atlanta Falcons on Sunday, and not have to think through another rematch against Seattle — even if it meant going on the road. The Seahawks and rookie quarterback Russell Wilson had more success running the football on the 49ers than any opponent, averaging 156 rushing yards in two games against their division rival. That number was far higher than the 94.2-yard average San Francisco surrendered throughout the regular season — and a reason that, the night before the Seahawks played the Falcons in the divisional round, Smith’s scouting report for Seattle sounded like this:

“Just not do what we did last time,” he said.

Not that Atlanta makes things much easier. The Seahawks torched the 49ers in their final meeting, but at least Wilson and Marshawn Lynch were the threats San Francisco knew. Defensive coordinator Vic Fangio hadn’t studied Atlanta closely until this week, and there’s mystery in an offense with such versatility. The Falcons’ three Pro Bowlers on offense are quarterback Matt Ryan, tight end Tony Gonzalez and wide receiver Julio Jones. The roster for Hono­lulu doesn’t even mention wide receiver Roddy White, who had 92 receptions for a team-high 1,351 receiving yards.

“You’ve got to have a good plan and a good mix to stop this offense,” Fangio said last week. “. . . They have good weapons all over the place.”

Gonzalez represents the most complicated of those, and the responsibility of getting in the future Hall of Famer’s way will be on inside linebacker Patrick Willis. Pro Football Focus ranks Willis as the NFL’s best at his position, and he’s particularly skilled in coverage. He has two interceptions, five deflections, and opposing receivers have gained only 355 yards against Willis all season.

This is the beauty of the San Francisco defense: specialists all around. Willis is the coverage man, Justin Smith is among the game’s best at stopping the run and Aldon Smith is an elite pass rusher.

The 49ers’ only flaw seems to be their secondary, and that’s not much of a weakness. San Francisco’s cornerbacks are strong in coverage, but the only thing they lack is takeaways. Tarell Brown and Chris Culliver each has two interceptions, not among the league’s top 30, though safety Dashon Goldson has added three picks.

The lack of dazzling statistics hasn’t dampened the defensive backs’ enthusiasm for Sunday’s contest.

“It’s definitely going to be a fast-paced game. A lot of running, a lot of balls in the air,” said Brown, who has deflected 12 of the 90 passes that have been thrown in his direction. “But it’s the type of game that we’re looking forward to.”

The 49ers’ defensive star power won’t just put pressure on Ryan and the Falcons’ offense; it’ll push Atlanta’s defense to play better than it has, particularly against multidimensional quarterbacks. The Falcons faced the Washington RedskinsRobert Griffin III only for two full quarters before he left the contest in Week 5 with a concussion. But Atlanta struggled in two meetings against the Carolina Panthers, allowing more than 400 total yards in both games, and watched Wilson ignite a second-half comeback in last Sunday’s NFC semifinal that befuddled Falcons defenders. Only two long passes by Ryan and Matt Bryant’s 49-yard field goal prevented an embarrassing loss.

But San Francisco sees itself as the one facing the most blowback if Sunday goes sideways. Justin Smith is healthy enough, despite a large brace on his arm, and his defensive teammates stand to benefit from his presence. So the talent is there, no excuses, and the memories of last year’s conference title game, a 20-17 overtime loss to the New York Giants, haven’t faded.

“That window for chances like this are slim, and we have to take advantage of the opportunity,” Willis said. “. . . We’ve been in this situation, and really just bringing it home. It’s not good enough to make the playoffs; it’s not good enough to make it to the NFC championship.

“We want to win it all, because that’s what we would be remembered by the most.”