It represents the steepest learning curve in football, and the lessons learned by Neil Lennon's group-stage rookies when they faced Benfica last night in front of a crowd of 53,373 were as painful as they were plentiful.
What is good enough for the SPL has to be improved upon immeasurably to even be adequate for the Champions League.
To a man, Lenny's learners grasped this truism, and will today reflect on how tough life can be when you move into the big boys' playground. The goalless draw extended to seven the number of opening group games Celtic have failed to emerge victorious from.
But it also matched their best-ever matchday No.1 result, against Aalborg four years ago, when the Hoops last competed at this level.
Celtic had played 18 home group stage games before last night, sending their fans home celebrating victory in 13 of them – including two against Benfica – and bemoaning just one defeat.
By contrast, Benfica were playing their 27th away game in the Champions League – with only four wins.
Yet, the bookies made the Portuguese champions – who had never even scored in three previous attempts at Parkhead, and still have not – warm favourites to start Group G with a victory.
Without Luisao and Maix Periera, two established stars in a team going through the pangs of transition, they were vulnerable, not least at left-back where they had to field inexperienced winger Enzo Perez.
It should have been the signal for Celtic to get the ball to James Forrest as often as possible to test the nerve of the kid.
However, just like at Perth last weekend, Forrest was peripheral to the play. More often than not, it went through the centre or down the Hoops left, where Charlie Mulgrew was operating as an auxiliary wide midfielder to supply the height and physical presence missing through the absence of Georgios Samaras.
If only Mulgrew could also have supplied the goal threat Sami supplies, especially in Europe.
The call to deploy Mulgrew in midfield was just one big decision made by Lennon.
His squad should be much more settled than that of Benfica at this stage, even though 15 players have left the club since last season, and five have been recruited.
But injuries continue to upset Lennon's best plans, with the loss of Gary Hooper to the Achilles knock he took early in Saturday's defeat against St Johnstone the final setback befalling him as he planned to try and get off to a winning start.
With Miku up front and Kris Commons attempting to support him, the plan was to blast from the blocks and try to catch Benfica cold.
However, the Celtic boss warned that starting at a high tempo is laudable, but requires to be rewarded with a goal.
Though Commons had a couple of efforts within the first few minutes, visiting goalkeeper Artur remained untroubled and untested, and this appeared to give the reshuffled Benfica pack the confidence to look to obtain a foothold in the game.
This they duly did, thanks to the clever play of the very experienced Pablo Aimar and Nicolas Gaitan's fast footwork.
With Rodrigo carrying an ominous threat up front – Fraser Forster had to rush out to block an effort from him when the defence were caught out by a ball over the top mid-way through the first half, a challenge which left Benfica shouting for a penalty – the caution the Hoops showed was understandable.
The shakiness of Emilio Izaguirre – a shell of the player he was before his ankle injury, and, like Victor Wanyama, booked in the first half – did absolutely nothing to instil a sense of calmness at the back.
But other than a near-post save from an Ezequiel Garay header after a whipped in corner caught Hoops napping, there were not too many moments of anxiety.
The introduction of Thomas Rogne as Benfica brought on top scorer and powerhouse Oscar Cardoza confirmed that Lennon was reactive and had done his homework on a side which reached the quarter- finals of this competition last season.
With Hooper on for Izzy midway through the second half, the idea that the Parkhead boss was content to settle for a point was without foundation.
Commons certainly got the message, as his audacious attempt to lob Artur from 50 yards underlined.
But, throughout the game, set-pieces represented Celtic's best chance to snatch a goal.
Ultimately, the experience of Benfica proved to be an insurmountable hurdle, but the experience of the occasion will stand Celtic in good stead for the next test, which comes up in Moscow at the start of next month.
By then, the hope is that at least some of the key men ruled out last night through injury will be back in situ, and new recruits are up to speed.
Last night's opening salvo will have been dissected and analysed in the continuing pursuit of qualification from a group which promises to be the acid test for everyone trying to earn their Champions League spurs.
