Federally in the US, only race, sex, and religion are protected classes, i.e. it's illegal to make employment decisions (hiring/firing/salary/promotion/etc.) on the basis of a worker's race, sex, or religion.
Let's say that I have a Christian employee, Joe, who verbally harasses a gay employee, Steve. Joe is confronted by management or HR about his behavior, but he defends himself by saying that he is a devout Christian and he is only attempting to save Steve's soul. (For the record, while I personally am a Christian, I don't think the workplace is the right place for these conversations.)
What could the company do? Could they reprimand, or even fire, Joe for his harassment of Steve? Or would the company be required by federal law to keep Joe on staff even when he harasses other workers?
I can think of an example involving only religion as well. If I saw another employee wearing a Satanic symbol I would be very offended. But Satanists have religious freedom too and are also protected by federal law. So who wins?