The Bundesliga side had won 2-0 at Celtic Park a fortnight ago and it looked ominous just after the half-hour mark when home captain Lars Stindl, who had opened the scoring in Glasgow, volleyed past Craig Gordon.
However, Dembele - who had a penalty saved in the 7-0 defeat by Barcelona - levelled from the spot in the 76th minute after defender Julian Korb was shown a straight red card for pulling the French striker down in the box.
Celtic, who will play in the Europa League if they can finish third in Group C, remain bottom, two points behind the German outfit, and face leaders Barcelona and Manchester City in their final two Champions League fixtures.
“We’ll keep believing we can continue,” Dembele told STV. “We’ll take it game-by-game and hopefully we can still be in Europe.
“I think we played much better football (in Germany), we believed in ourselves and we had a lot of chances.
“We deserved better in the game. We keep learning and hopefully in the next one we can get a result.”
Celtic boss Brendan Rodgers complimented Dembele on his composure from the spot.
He said: “It was a big moment for him and I am sure (the miss in Barcelona) went through his head but he showed great composure.
“He is a very good penalty taker and we have a number of them and he showed great composure against a top-class keeper and set up the last 15 minutes.
“I thought he was immense. For a 20-year-old boy to perform like that, he started loose but his presence, quality and strength really disturbed their back four.”
However, Rodgers was left ruing substitute Callum McGregor’s late miss which would have put a different complexion on the group.
“We should have scored. He has either got to score or square it to finish,” the former Liverpool manager said.
“I thought some of our link-up play to create the possibilities was excellent.
“He has arrived into the box and had his shot but unfortunately for us it didn’t go in.
“It is very important to create the opportunity but that was probably the moment we felt we could win it.”