Strikes either side of half- time from Kris Commons and Georgios Samaras had given the Hoops parity with Shakhter Karagandy, and one foot in the door of today’s draw.
But it required 22-year-old Forrest to get them over the line as he connected with Anthony Stokes’ cut-back to send the ball high into the net – and the Celtic fans high into orbit.
The scenes of celebration in the stands and on the pitch after the final whistle blew were reminiscent of the delirium which engulfed the stadium after they had won a penalty shoot-out against Spartak Moscow to book their place in the 2007 group stage.
The 3-0 win means that Neil Lennon and his players are back among Europe’s elite.
Had they failed, it was the Europa League – and only a tenth of the money Celtic will earn in the Champions League.
That would have had a significant impact on what happens between now and the transfer window closing on Saturday night.
Celtic have already banked £2m for taking part in the qualifiers, and are ready to add Israeli midfielder, Nir Biton, to the four players they have already signed this summer.
The 21-year-old could only have been impressed by the determination and endeavour shown by his new team-mates last night as they more than made up for the 2-0 defeat suffered in Kazakhstan last week.
It was always going to be a night with a real edge to it.
Within 20 seconds, Lennon was out telling the fourth official that giant striker Sergei Khizhnichenko had barged Efe Ambrose while the ball was in flight, perhaps remembering how that tactic had worked so well for Mo Bangura when Elfsborg played the Hoops in the previous round.
But sometimes Bhoys have to be left to fight their own battles – and this was one of those occasions.
Unlike in the first game on the plastic pitch in Astana, when it took 26 minutes for the first foul to be awarded, this was a real contest on a real surface.
Testifying to this was the fact that, by the same stage this time, 10 fouls had been committed, eight by Shakhter, who had three men booked by half-time, Gediminus Vicius, Aleksander Simcevic and Roger Canas.
Joe Ledley and Igor Zenkovich joined them in the second half.
Surprisingly, the Kazakhs – who were expected to sit deep in banks of five and four – pressed high up the pitch in the early stages, forcing Celtic to go long.
This did not suit the Hoops, and decision-making was driven by anxiety, so passes went astray as doors closed in their face.
Early chances, in the form of headers from Mikael Lustig then Anthony Stokes, brought saves from Aleksander Mokin before a Commons free-kick saw the keeper fly across his line to paw the effort away.
But, just like last week, he looked more athletic than technically adept – or safe –which gave the Hoops hope.
Forrest and Stokes were off target when they got a sight of the Shakhter goal shortly before the interval, while the nearest the Kazakhs came to grabbing the vital away counter came from the boot of Khizhnicenko.
But his low shot after eight minutes had been cleanly gathered by Fraser Forster, who also saved well from Simcevic just after Celtic got their second goal of the night.
The longer the game went, the more confident the Kazakhs became that they could frustrate the life out of Celtic.
Commons came up with the perfect riposte, blasting home from 30 yards just seconds after the referee had indicated there would be two minutes’ of stoppage time played at the end of the first half.
Collecting a pass from Samaras and changing the direction of the play, the man who fired Celtic into the last 16 with his spot- kick against Spartak Moscow last year raised the roof once again with his howitzer which flew in at Mokin’s right-hand post.
The cheering and celebrating did not abate throughout the interval, while, deep in the bowels of the stadium Lennon was exhorting his players to keep calm – and vigilant – in their pursuit of that all-important second.
Within three minutes of the restart, they had levelled the tie.
The Hoops’ ninth corner was cleared only as far as Lustig 20 yards from goal. His shot struck a defender, but fell to the feet of Samaras who took one touch to control the ball then another to send it past the keeper and into the net.
Had Stokes found the target instead of the crossbar on the hour mark after Forrest had picked him out perfectly at the far post, the turnaround would have been complete.
The painful reality was that a goal for Shakhter would put them back in the driving seat. And it was not surprising that nerves appeared
to set in.
Adam Matthews had to boot an Aidin Dzidic header off the line after Forster’s punched clearance from a corner had fallen to him, then panic ensued as a throw-in was allowed to bounce in the six-yard box allowing Khizhnichenko to touch a shot off the bar.
The fear of letting it all slip was shared by players and supporters alike, causing the earlier momentum to be tethered.
With Derk Boerrigter introduced in place of the tiring Commons, Lennon clearly wanted the job done in 90 minutes.
In stoppage time, up stepped Forrest to oblige – and spark pandemonium in Paradise.