How Do We Know if Something Is True?

How do we know what we know? If we're going to claim something is true, or something is valid, or that it is this way, how do we know that? Here some vor questions from pistemology. How can i know anything for certain? What do i know for certain? Of all the stuff i know, there's various degrees of certainty i have for it. Is there anything there that i'm a hundred percent certain about? So what makes justifications valid? Also, why do billions of people believe in god? A lot of religious people and also, atheistic people dismiss this question waig too easily.