There are many more religions in the world. They are all too many to describe in a book about non-belief.
Christianity and Islam both state that they teach the Only Truth and that all other religions are false. Hinduism claims that there are many truths, but that you are definitely reborn when you die.
All can’t be right at the same time.
There is no way to find out which religion is true or false without dying first. How else can you find out if Jesus was God’s son, Muhammad Allah’s last prophet, or if you will be reborn as a donkey?
The problem is that if one religion is true, then the others must be false. You don’t have to be a non-believer to understand that – the religions say so themselves.