But they aren't allowed to fire anyone for their religion. Hence, the reason for the suit.
Sure they can. There's tons of ways an employee can be fired, disciplined, etc for practicing their religion.
Title VII only allows for some basic protections & there are many ways an employer can circumvent those.
But he wasn't fired for being religious, he would never have gotten the job, he was fired for expressing an opinion. Not saying I approve of the firing, but it's not saying "I approve of this religion" or whatever. And my larger point is that I think private employers SHOULD be allowed to hire or fire employees for any reason, not that this is the law.