Why are Christians so confident in having the “one true faith” when so many other religions are just as old and deeply followed?

If so many other religions are just as old, deeply followed, and believed in around the world why are Christians so confident that theirs is the only true one? What makes them so sure everyone else is wrong, especially when most people just believe what they were raised with?
Why are Christians so confident in having the “one true faith” when so many other religions are just as old and deeply followed?
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