The man who does not do his own thinking is a slave, and is a traitor to himself and to his fellow-men. If anyone can show me, and prove to me, that I am wrong in thought or deed, I will gladly change. I seek the truth, which never yet hurt anybody. There is nothing which can better deserve our patronage than the promotion of science and literature. 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.