They were within seconds of taking a point off one of the best sides in the world, but the big thing for me was how well they worked as a team on the night.
The confidence they had in themselves was there to see, and they went into that game believing they were capable of getting something from it.
That says an awful lot about them as a team – and it also bodes well for the future.
Another huge thing for me was their fitness levels. To chase and restrict the space for Barcelona in the manner that they did suggests they are a group of well-conditioned athletes.
And although it will take them a few days to absorb the disappointment of coming so close to getting a result, what you can say is that not once in the group stage of this competition have Celtic looked out of their depth.
There is a growing maturity not just to the team itself, but also from Neil Lennon. I said it a few weeks back, but I really do think that he has grown in stature as the Celtic manager.
He got his team exactly right on Tuesday. He tried to force Barclona wide all the time, but as many teams have discovered, Barcelona just keep on coming at you.
When you weigh it all up, Barcelona were worthy of the win. Their levelling goal was a work of art, and although you can try and defend against football like that, sometimes you have to take your hat off and applaud how well crafted a move can be.
It will be just as difficult a game for Celtic at Parkhead next month when the Catalan club come to town. Just as they did in the Nou Camp, they will have a lot of the ball.
Celtic will have a massive crowd behind them but, more importantly, they will have a great deal of confidence going into the game.
They will have respect for Barcelona and the ability they have, but they will not have any trepidation.
Neil mentioned that they were brave in the Nou Camp and I'd agree with that. They will need to show that same courage on the ball again in Glasgow, but I think they will have a lot of belief that they can get a result.
From my own playing days, what I recall most when we were playing top opposition is that you need to show the right attitude, the right application and the right concentration levels; that still holds now.
Against Barcelona, Celtic ticked all those boxes and, in terms of an all-round team performance, I thought it was excellent.
It was always going to be a three-way fight for second spot. Despite the Barcelona defeat, getting second is still well within Celtic's grasp.
But the one thing I would say is that you wouldn't want to be getting too far ahead of yourself.
Once the next game is out of the way, Celtic will have a far better idea of where they are and what they have to do.
At the minute, though, they need to keep their focus on the next game.
If they can produce the same level of performance when they play Barcelona at Celtic Park then they could get themselves a point, or even more, but it is pointless to make a guess just now when there are three remaining games still to come.
Celtic will take confidence, not just from the way they performed against Barcelona, but also from the way they have acquitted themselves in Europe this season.
They are growing in maturity as a team and the results they have had in the qualifying rounds and then in the group stage both quite clearly show just how far they have come as a team.