On a night when the Scottish club celebrated a milestone of longevity, its disciplined defensive display repelled Europe's best team of recent times as mighty Barcelona fell to a rare 2-1 loss Wednesday in the Champions League.
"I think it was one of the greatest nights in the club's recent history," Celtic manager Neil Lennon said.
Victor Wanyama headed the Scottish champions in front from a corner in the 21st minute and teenage substitute Tony Watt rounded off a late break forward to score the decisive goal.
"The players are heroes, they are heroes to me," Lennon said, to the backdrop of cheers from fans celebrating into the night outside Celtic Park. "I can't speak highly enough of their performance tonight and they will go down in the history books of the club as the team who beat probably the best team in the world."
Celtic had also been leading against Barcelona two weeks ago in the Camp Nou. But Celtic's dreams were shattered by two home goals, the last in stoppage time, as they lost 2-1.
Again Celtic conceded in the 90th minute on Wednesday, but Lionel Messi's goal was too little, too late, after being continuously thwarted by goalkeeper Fraser Forster.
"It was the perfect result to mark 125 years since Celtic was founded," Lennon said. "One, to play Barcelona in such a prestigious game and two, to win the game under huge difficulties with the squad decimated by injuries and illness."
Barca's first loss in Group G after three straight wins prevented them from securing early qualification for the knockout phase.
"Before we played I knew they were good at set pieces and we didn't want to concede too many and we didn't give them too many chances," Barcelona coach Tito Vilanova said through a translator. "But I can't be satisfied because we want to win the game.
"It is a really difficult stadium but we were like the home team and we locked them in the box."
The loss ended the Catalan side's 12-match unbeaten run.
"It is not the first time we have lost a game like this and it won't be the last," Vilanova said.
Celtic, which won its only European title in 1967, is now just two points behind Barcelona with games at Benfica and at home to Spartak Moscow remaining.
"We still have two very tough games," Lennon said. "The problem is the players and staff will have to deal with is that the expectation level will increase. There is still a lot of work to do but we have given ourselves a platform."
Barcelona took just eight minutes to put Forster under pressure with Xavi Hernandez powering in a shot from outside the penalty area.
But Celtic was crowding out the visitors in defence and when the hosts finally found the space to launch a counterattack they delivered.
"We knew we could affect them at corners," Lennon said. "We looked at their back-post area with (Jordi) Alba in the zonal system and Charlie Mulgrew is one of the best dead-ball players around."
Mulgrew's delivery was met by Wanyama, who rose above Alba to head past Victor Valdes.
"The first goal was important and gave us the energy to go on," said Nigeria midfielder Wanyama. "We have been working hard at set pieces, we knew we were going to get one and we did it well."
Barca couldn't always get the space to find its rhythm, but looked dangerous when they did.
An interchange with Andres Iniesta after beating three defenders saw Messi break clean threw, but his effort was tipped over the crossbar by Forster.
The visitors weren't panicking in the search for an equalizer, with the passing carousel around the penalty area patient before Xavi found the space to launch a shot that couldn't match the quality of the buildup.
Alexis Sanchez was denied by the post before halftime from a header and, as Celtic continued to pack its defence, Forster was Celtic's assuring presence between the posts.
The English goalkeeper kept out a dipping shot from Messi and pulled off a double save to deny Alexis, diving low and then using his feet to block the follow-up.
Forster did even better to deny Messi from point-blank range, palming the ball away with a diving save after the Argentina forward's one-two with Jordi Alba ripped through the Celtic defence.
And there was an easy save to make when Messi sent a tame free kick straight at Forster.
Celtic went straight up the other end to score.
Forster's clearance went high over Xavi and fell straight at the feet of 18-year-old striker Tony Watt, who lashed the ball straight past Valdes.
"I just saw Valdes, I thought to put it to the side of him and it might go in," Watt said. "It's probably the best moment of my life."
Barca never gave up hope of a second comeback in two weeks, but even Messi's late goal proved to be too late.
Forster was finally beaten only after he denied David Villa and Messi pounced to send the rebound into the roof of the net.