The former Dundee goalkeeper arrived at Parkhead as an emergency loan signing during the winter transfer window following a long-term injury to Craig Gordon, and he was thrown into the deep end of Old Firm action after back-up goalie Dorus de Vries was ruled out the morning of the game.
Bain came out with his head held high in the 3-2 win over Rangers in March, and he admitted to Celtic's official website that his heroics in the encounter secured him a permanent move.
“I’m delighted. It’s more than I obviously expected when I first came in. I’m over the moon," the 28-year-old Scotland international told Celtic TV after agreeing the new deal.
“I don’t think it could have gone any better. It’s funny, there’s a lot of little things that went my way to get to Celtic on loan and then to play the game against Rangers, which sent me on my way to this four-year-deal.
“That game showed the manager everything he needed to know. He said after the game that it’s one of the biggest games to play and it really showed him my mentality and that I could handle being a Celtic player."
Bain's impressive debut, with 10-man Celtic coming from behind twice to seal an emphatic victory, was enough to convince all of his quality.
He has gone on to make five more Scottish Premiership appearances, keeping three clean sheets in the process.
It remains to be seen if Bain can pip Gordon to the starting berth, but he remains a capable and quality backup who can step up whenever the need arises.
His presence will also keep the 35-year-old on his toes, and he has all he takes to become Gordon's long-term successor.
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