@TangoFoxtrot there is a difference between workplace bullying and harassment, and discrimination. Take this behaviour to HR.

This answer seems to confuse harassment and discrimination. Even if discrimination were both legal and allowed according to company policy, harassment presumably still isn't. (IANAL). You might want to remove the first part of your first sentence.

I have hired and fired in both Georgia and Texas (the deeply religious state in the United States). I am a currently a hiring manager myself. This harassment situation described by the OP is absolutely appalling. Evaluate the person on their work. Get rid of those toxic to your team. I'll use a different example. Let's say in my religion I don't believe in divorce. I learn that my co-worker has been divorced before and just insult him for it. Honestly, how does the fact someone's been divorced even matter if they are performing the job you hired them for?

@TangoFoxtrot Helen is being toxic, Mark is not, why would you get rid of Mark? Toxic people are rarely toxic to only one person. What if she finds someone else she has a problem with? "Get rid of everyone Helen has a problem with" is a terrible long-term strategy.

@TangoFoxtrot: seeing someone harass someone else makes me feel really uncomfortable, and I wouldn't want to work with the harasser, even if they left me alone. The reason is that I could be the next victim. Plus, where I am now, the project manager allows people to shout bs against women. Guess who is searching for another job. And this is just an example on how someone might react on "just" witnessing harassment.

@Dukeling Helen is only "toxic" to Mark. If she were that "toxic" she's also harass others and she doesn't.

@TangoFoxtrot - if you get rid of Mark (or otherwise remove him from the situation in a way that is adverse to his career) for being bullied because of his sexual orientation, that is discrimination. If you get rid of Helen for being a bully, that is not discrimination. You are not getting rid of Helen for her religious beliefs, you are getting rid of her for being a bully. You say she is not toxic to anyone else - but what happens when someone else falls foul of her personal judgment?

It's easy to say this and that, but if Helen is fired she will, of course, instantly litigate because she could trivially argue that she was fired because she is Hasidic / Sunni / Amish / whatever the heck she is.

Keep the religion out of it. Strictly insist on that (also to yourself - I am concerned you mentioned that in the post, it should not even be there). Most religions have a clause that permits them to follow the laws of the land. ("redde caesar quae sunt caesaris" - give Caesar what belongs to Caesar) And so, in minor form, that is valid in a company. Outright insults are easy to identify, and to deal with. Zero tolerance for that. Period. This is the easy case (I had to handle much more subtle ones), and there is no excuse not to do it.

@HorusKol "but what happens when someone else falls foul of her personal judgment?" - Same thing that happens when someone else starts insulting Mark in Helen's absence. We cross that bridge when we come to it.

@CaptainEmacs How do I keep religion out of it when Helen's entire argument is based on her religion?

@TangoFoxtrot: Ideally, by ignoring her (invalid) argument. Her religion doesn't give her the right to harrass a coworker without consequence. Nothing does.

@TangoFoxtrot You mention that you are the "new manager". One of the central skills you have to learn as a manager is to ignore pseudo-arguments. It is completely irrelevant what her religion, favourite sports team, political party or whatever affiliation is. She is not allowed to taunt, harass, insult her colleagues, full stop. You have to put the foot down. Keith Thompson has it right. Your own belief is, in consequence, equally irrelevant for the steps you have to take. In fact, ideally it should not appear in your question.

Difference is, sports team affiliation and political affiliation are not protected classes. Religious affiliation is.

@TangoFoxtrot: Yes, her religious affiliation is protected. She has exactly the same responsibility to refrain from harassment as anyone else has, regardless of her religious beliefs.

@TangoFoxtrot - it seems to me that you have no actual desire to do the right thing and are quite happy to continue allowing the bullying behaviour - everyone here so far agrees that Helen is wrong, and yet you keep arguing in her favour. Did you come here for answers, or validation?

@TangoFoxtrot: if having a bit of empathy and being able to see possible consequences of bad behaviour makes me be a sensitive person, then yes, I am sensitive, and proud of it. I'm sorry you live in a place with so much lack of this positive trait that you see it as an abnormality, and I mean it.