Knowledge is in every country the surest basis of public happiness. Religion is harmful because it claims knowledge of an absolute truth about what it is to be human. By definition such a claim is dangerous. It leads to immoral decision-making and excludes new insights into what it is to be human. Religion blocks the development of rationalism as the guiding principle of human development.
Religion hinders the development of scientific knowledge.
Religion gives a distorted view of reality by claiming false things to be true. The faithful suffer stress because the evidence of their senses and experience conflicts with their relious beliefs. Religion creates and reinforces rifts between different communities. It spreads from generation to generation like a computer virus and perpetuates these rifts over many generations. Vices develop when people are forced to keep different aspects of their humanity in separate compartments.