Christie put Celtic 3-2 ahead against Cluj in the 76th minute but the brilliantly-worked goal was undone inside four minutes when the Romanians worked space to shoot before Billel Omrani netted from the rebound.
The final blow came after six minutes of stoppage-time but the damage had already been done with Neil Lennon‘s men heading out on away goals before the visitors snatched a 4-3 win.
Celtic had earlier bounced back from a tame first-half display to overturn a 1-0 deficit just after the hour mark but Scott Brown’s split-second of panic at a corner led to a crazy decision to handle and allow Omrani his first equaliser from the spot.
Christie said: “We need to be more street-smart, especially going up twice in the game and not really managing to see it out. It’s a very bitter pill to swallow.
“Everyone is absolutely devastated. Everyone wants to play Champions League football at this club, everyone thinks we deserve to be there but you can’t concede four goals at home and expect to go through.
“It’s all very raw. Over the next few days we can look back and analyse it a bit more and try and learn from it, but when you go a goal up, especially in Champions League football, especially when you are at home, that should be the tie over.
“We wanted to keep going and get a bit more of a cushion but at the same time we need to keep the back door shut and we didn’t do that.”
Christie’s goal was his eighth of the season and he was aiming to take his form on to the biggest club stage.
The 24-year-old was unable to break into the team when Celtic played in the group stages in Brendan Rodgers’ first season, and was on loan at Aberdeen when they qualified again the following year.
And the former Inverness man could not hide his disappointment at having to settle for another season in the Europa League – if Celtic come through a play-off against AIK or Moldovans Sheriff with the Swedes 2-1 up ahead of their home leg.
“Since I broke in last year, me more than anyone else, I was desperate for Champions League football this season,” Christie said.
“Coming off a treble-treble, you put in all that hard work to hopefully go and push on again the next season.
“I think personally we are a Champions League team but you need to prove it on the pitch and get through these tough ties. Fair play to Cluj, they have come here and managed to out-smart us.”