Didn't realize the age was mentioned, it's a bit late! That definitely does make it different, however I still stick with my original advice. Sometimes Church is just about the experience. I'm agnostic myself with an atheist father and Christian mother (divorced for irrelevant reasons). I was raised by my mother who brought me up as Christian. The few times she forced me to go to church (alone), they were Baptist churches. I told her I didn't like them and they freaked me out, so she said cool and I didn't have to attend. I respected that.

I later went to a Catholic church of my own choice because I wanted to see what it was like out of pure curiosity. The building was beautiful and the children looked like angels and it was completely different from the fire, hell, brimstone preaching that I'd been exposed to previously. I kinda liked it. I learned absolutely nothing about the word of the lord but was sincerely impressed with their band that they had and how nice everything looked.

As such, I think if he wants to take the children out on a religious holiday (Easter) to dress up in their nice clothing and see the pretty kids and look at the beautiful stained glass windows that it's more reasonable than him preaching at them and forcing them to go. I mean it's not like they'll be able to comprehend what is going on at >3. He definitely needs the wife's permission however and shouldn't just outright disrespect her by taking them anyways. I just think from experience that taking them to a nice Catholic church would be a lot more reasonable than forcing them to go to a more extreme/less cultured place.

I do agree wholeheartedly though that it is way too early to impose a religion on them that both parents aren't on board with. However keeping the previous agreement won't work and will just lead to more bitterness and arguing. The previous agreement was made when the husband was non-religious as well. He suddenly has decided he is a man of faith now, and while that sucks for OP, he has changed and marriages are about compromise and change. One member of the agreeing party has done a full 180, so she now has to compromise and meet him halfway in order to accommodate this.

For better or for worse, right? Those are the vows. Yes it's unfair but it's here and she can't just dig her heels into the sand and stick to everything determined years prior if she has any intentions of salvaging the marriage. I think if she agreed to something like allowing him to take them if they were fully willing so they can experience it WITHOUT him preaching to them/telling them religion in a factual manner (i.e. God created Earth 100%) instead of "We don't know how the Earth was made, but some people think this and some others think that and there are some ridiculous people who think the Earth was flat so it's really all up to you, kiddos." I mean it's great she wants them to be neutral but I think trying to prohibit them from hearing anything religious is sort of enforcing her atheism on them. I don't know, as long as they are not exposed to anything adult/"hell" and such kind of concepts, I don't think it is harmful for them to go to Mass or religious things.

As much as it sucks and isn't fair on OP, she cannot change it, and either she will compromise and be the bigger person to save her marriage, or she will shelter her kids from the religion, divorce, and have shared custody with the husband who is free to tell them whatever he wants while alone. Again I agree with you wholeheartedly and even said "allow him to say the word of God, but also discuss other religions." However neutral isn't sheltering them from all religious acts, neutral is sheltering them from anything that 3 year olds don't need to be hearing ("Burn in hell" discussion, "God 100% doesn't exist", "God 100% exists") etc just very absolute terms that affirm one person's position.

I'm a bit sleepy so probably not articulating this well but tl;dr I agree with you but also think that OP should stop fighting to keep her kids away from the Catholicism so much and instead focusing on the neutral language and her relationship issues/barriers without worrying so much about the children at this stage. They're >3, they will not remember this or fully understand the context of what is happening at all, will probably just enjoy the pretty lights and be a bit rowdy and try to sing if anything. Instead she needs to focus on her husband, him respecting her, his lifestyle change and the boundaries she has imposed.