They thought they had pulled off something remarkable – just about everyone did – but, three minutes into stoppage time, perhaps the greatest side in the world pickpocketed them. They scored a winner which deftly nicked away what would have been one of Celtic's great modern results and the finest of Neil Lennon's career as manager.
Adriano crossed and Jordi Alba got in front of James Forrest to score at the back post, a lapse punished mercilessly. Clinical and merciless. It didn't make Celtic's overall defending and performance less admirable but it denied them the chance to say they had come to a football cathedral and left on equal terms with the gods.
Celtic had pressed Barcelona into conceding an own goal and, even after Andres Iniesta's glorious equaliser, they refused to fold. For 26 minutes, Celtic even enjoyed the fantasy of leading in Camp Nou . . . plenty of time to take photographs of the giant scoreboards. Lennon didn't want Celtic to act like tourists but he would allowed himself a quick look to put that in his memory bank.
Still, his team remains second at the halfway stage of Group G, with Barcelona at Parkhead their next European fixture on November 7. Benfica and Spartak Moscow have a fight on their hands.
It was a night when Barcelona's vulnerability, as well as their excellence, was evidence. The defence which conceded four against Deportivo La Coruna looked unsure of itself when Celtic had a go at them from set-pieces. If only Celtic could have got a bigger foothold in the game to ask them more questions, but that is a little bit easier said than done. Celtic might have had a goal in the second half too when Efe Ambrose and Victor Wanyama both rose to connect with a Charlie Mulgrew corner in the goalmouth, resulting in the ball going wide.
What a crowd of 77,781 witnessed on a warm, sticky Catalan evening was a familiar story of Barcelona dominance and then being denied right until the game's final breath. Any visitor to Camp Nou feels obliged to purr over Barcelona's excellence. The certainty of their near monopolisation of the ball and masterful distribution was duly exhibited. The relentless percussion of their attacks can mesmerise teams and Lionel Messi, Iniesta, Xavi and their high-octane supporting cast swarmed towards Celtic for most of the night.
Technique, speed, power and intelligence, they have it all, but there were long spells when Celtic withstood them. Alexis Sanchez might have scored very early, Marc Bartra had a couple of good chances with headers and Adriano rifled a low shot to the corner which Fraser Forster did well to block, but the goalkeeper was not overworked. Messi put a couple of free-kicks just over the bar.
Barcelona had scored five in a couple of games this season, and four in another, but Celtic were too well-organised and motivated to suffer a similar fate. Their gameplan was straightforward but admirably executed.
The back four was protected by a tireless midfield four of Mulgrew, Joe Ledley, Wanyama and Scott Brown. It was a wall of green and white against which Barcelona's attacks lapped like waves. Messi, Pedro, Alexis and Jordi Alba got in behind them sometimes – of course they did – but Barcelona's patience was tested by the discipline and diligence of Celtic's play. Their deft little passes were intercepted, or tackles made to break up their attacks. Celtic tired towards the end, but still grafted.
Such is life for Barcelona. Every team turns up at Camp Nou simply hoping to contain and live with them. Their patience may have been tested last night but the certainty of their own brilliance prevents their play being compromised by frustration. Sure enough, they played their way to an almost inevitable equaliser just before half-time.
It was an ensemble piece by their three great artists. Messi played a firm low pass to Iniesta and his one-two with Xavi was a work of deadly quality. Suddenly Iniesta was in, and the man whose goal won a World Cup final wasn't likely to miss. He drilled a finish low inside Forster's goal. It was a goal of brutal brilliance.
Celtic's moment had come in the 18th minute, and what a moment. They won a free-kick on their right and Mulgrew planted a fine cross on to Georgios Samaras's head. His downward header glanced off Javier Mascherano's shoulder and flew emphatically inside Victor Valdes's left-hand post. Celtic's celebrations – on the pitch, at least – were relatively muted but their supporters, high on one tier and dotted elsewhere around the great oval, erupted.
On the touchline, Lennon kept his head. He bellowed at his men, pointing at his temples in a clear message: "think". It was a plea for them to remain cool and continue their commitment to the gameplan. They did so, even allowing for Barcelona's predictable recovery, but the loss of Samaras – he injured his ankle in an awkward fall just before half-time – was a grievous setback. He had been excellent even if his time on the ball was understandably brief. They lost Brown too, the captain departing after an hour having faded after being hurt in a collision with Messi.
Barcelona pressed, pushed and probed. Forster saved brilliantly twice, from Messi and then Pedro. Iniesta flicked a ball over the defence and Messi tried to bury a header but Forster was a match for that as well. The danger continued to the very end – David Villa hit the post in stoppage time – and it looked as if the defiance would as well. But no. Barcelona would not be denied.