Infact if you look into it you'd see that the vast majority of people of other faiths do not claim to 'know' and they just guess their religion is true... I've put this to the test and hard-core evangelical Christians have admitted they cannot be absolutely certain of the Truth of their faith... the Quran says that non Muslims just follow conjecture
Muslims are the only people who claim to know without a shred of doubt that's because they're the only people who have really witnessed the Truth...I.e. They have the light of faith in their hearts via which this truth becomes manifest
Well to be fair we cannot know anything 100%
And there are many people of other faiths that claim to know that their gods are real , not just the popular faiths now , but throughout history.
People don’t guess that their religion is true ... they believe it is true , for a variety of reasons,
You believe your religion is true , just like Hindus believe there’s is true , just like the Jews and Christians believe that theirs is true , as the ancient Greeks , Romans , Egyptians , Mayan’s , Incas .... they all believed that their gods were real .
They were all wrong ... given the fact humans have been inventing gods and creating religions .... don’t you think it’s possible that your god and religion is equally an invention