If, around Halloween time, an employee requests permission to hand out bags of candy containing “gospel tracts,” which depict Muslims and Catholics and state that they should all go to hell, you just go ahead reject that religious-accommodation request.
(More on religious accommodations here)
Cuz even though that employee may sue and have her case make it one step below the United State Supreme Court, she will lose.
While this is definitely an extreme example, employers should be careful not to favor one religion over another. However, they are not required to permit proselytizing in the workplace, especially when the message includes bashing other religions.