There are many religions in the world other than Christianity. The followers of those religions are just as convinced that their religions are true as the followers of Christianity are convinced that theirs is true.
But even if you’re not interested in religion, there are many different philosophies of life that can substitute for religion. The followers of each of those are also convinced that their particular philosophy of life is true.
Given all that, given this wide range of competing religions and philosophies of life, why should anyone believe Christianity? If it’s just one among many, how can anyone really believe it?