onyxelle, yes, it can be harmful for people to believe in implicitly in the God of the Bible, because they must reject Truth in order to believe Biblical lies, and often have a nasty tendency to try to impose their ridiculous beliefs on others.
Whether it is knocking on doors, passing out religious tracts, getting rules of their religion codified into laws, making schools teach religious myths in science class, subordinating women, outlawing medical research, or telling other people the "right" way to think and live, your religion affects us all.
How many cultures were destroyed by missionaries? How many lives by the Inquisition, slavery, countless religious conflicts and the Bush Crusades? How many minds by the demand that they reject science and believe the illogical tenets of Christianity? How many women by being limited to "Kinder, Kuche, Kirche"?
If all Christianity did was to encourage people to be nice to each other, it would be fine with me. But religious fanatics can be the most close-minded, arrogantly ignorant, intolerant, smug and self-righteous people I have ever encountered. Not all of them, of course - I know many good Christians who keep their religion to themselves, as they all should.