Dean be prepared for her to start yelling religious freedom and persecution crap.
If she does this in front of the kids remind her that religious freedom works both ways and remind her again you will not allow her to bully, frighten or force the children to go to the meetings. They are old enough to choose. Some get baptized younger than they are.
Remind her too that God wants people to love Him not fear Him.
As much as possible do not have these arguments in front of the children. If necessary ask them to go out to a friend's house while you and the missus talk about this.
Let the children know you are there for them and have no intention of leaving them. And let them know you will listen to what they have to say if they are troubled and they probably are.
When you and the missus settle down and can talk reasonably see if you can arrange a family meeting. Let the kids know where you stand. Do this in front of her. Do not create secrets and say things behind her back. Be upfront and honest. They are all old enough to deal with this - they are right in the middle. They need information to help them feel secure as possible in all of this.
Do not put her down or her beliefs. Just acknowledge they are different than yours
Sort out for yourself if you can manage the kids on your own and how you would do that. Be prepared when you go to the lawyer. Find the kids birth certificates, your marriage certificate, home ownership papers, cars bank accounts any savings, etc. Get these now before she does and put them in a safe place