WHEN Rasmus Schuller capitalised on defensive indecision to put HJK Helsinki one-up against Celtic at Parkhead on the first night of this month, the response from Neil Lennon's side was instant – and decisive.
Gary Hooper fired the Hoops level within six minutes, and Charlie Mulgrew popped up seven minutes later with what turned out to be the goal which gave them a lead to take to Finland, where Celtic cruised through to the play-off with a 2-0 win.
Should Lennon's side lose the opening goal tonight when they line up against Helsingborgs in the second leg of their knockout tie, the nerve will have to be even more unwavering, the resolve to bounce back even stronger.
In essence, this is the biggest test of character the players and manager will ever have faced, and failing will leave an indelible mark on their careers.
Conceding the first goal, and thus throwing the Swedes a lifeline, is a prospect Lennon has astutely included in team talks, attempting to mentally place his players in that position so it does not leave them stunned should it actually happen.
His counterpart, Age Hareide, is doing all in his power to paint it as the doomsday scenario for Celtic, playing as they are with one foot already in tomorrow's draw for the group stage.
Like Chris Tarrant on Who Wants To Be A Millionaire, the £21million cheque is almost in their hand, only for him to whip it away with the words: "But, I don't want to give you that."
What Tarrant does want to give contestants is an even bigger cheque. And, what Lennon wants to give his players is an even bigger thrill than qualifying for the group stage.
The Hoops boss is desperate to let them savour competing in the competition proper against some of the world's biggest names.
He is confident that, with the addition of another couple of players – who will be invited to join the club as soon as this tie is over – they are good enough to go toe-to-toe with the game's giants.
But, first, they have to prove they are good enough to book their ticket to the big time, that they have the mental and physical attributes to cross this final hurdle.
Helsingborgs will draw on the fact they won 3-0 in Poland against Slask Wroclaw in the previous round, and that a repeat of that scoreline would see them record the biggest upset since they eliminated Inter Milan to reach the Champions League 12 years ago.
Celtic are determined not to become another chapter in the Swedish club's folklore. To this end, Lennon will spend every available minute between now and kick-off calming nerves and curtailing complacency.
He will be on guard against any sign of anxiety or tension, but knows it will not be until the whistle blows that he will see clearly who can handle the occasion, and who might struggle.
The manager said: "I'll gauge any nerves after the first 10 or 15 minutes of the game.
"But I am sitting here much more comfortable than I would be if it had been 0-0 or 1-1 in the first leg, or we were chasing the game. They know the tie is not over yet, so we will be as positive as we possibly can."
In an attempt to further defuse any building pressure, Lennon rejected the suggestion this is the most important game of his managerial career, propelling winning the title into that position instead.
That is true in so much as, without that title win, he might not have had the chance to lead the club into this Champions League qualifying campaign.
But today he stands on the threshold of getting them back to the group stage for the first time in four years, and can be proud of the way they have negotiated the qualifying minefield to this point.
If they can take this final step, the door will be open to squad strengthening, with Lennon re-iterating that a central defender, a striker and a left-sided player are on the shopping list.
Daryl Murphy will head to Ipswich after tonight's game, while Mo Bangura has already left the club, moving back to AIK Stockholm on a year's loan.
That will make room for the likes of Filip Twardzik and Tony Watt to continue their push for first-team involvement.
But, as he compares his squad to what Celtic could meet in the group stage, Lennon accepts they remain in the lightweight category, and today's fitness tests for Scott Brown, Kris Commons and Thomas Rogne will be key.
"If we do get through, in terms of financial muscle, then we would be punching well above our weight," he admitted.
"But, if we can get to the group stage, then that's progression, which is what you are always looking for.
"I'm not getting ahead of myself though. We still have to qualify against a very good, highly- motivated Helsingborgs team. This is big, and football has a funny way of kicking you when you think you've cracked it.
"We have prepared as low-key as we possibly can because we know there is a lot of hype surrounding this game. We've tried to protect the players from that and, really, we just want to get the game started."
And the tie finished.