Dear (insert ex-husband name here):
I am glad to hear of your newfound or return to religious fervor. I am hopeful it will help you overcome many of the personality weaknesses that led to the collapse of our marriage.
I am hopeful however that you will not be bringing up religion in our divorce nor custody hearings as it is irrelevant to what is in the best interests of our children. What is best for our kids is what matters most to me.
As it seems you are interested in making this about religion, I am prepared to agree on the following regardless of what a court decides about who becomes the custodial parent:
1. Both parties agree not to denigrate the other parent or that parent's held beliefs in front of the child nor allow the child to become aware of such comments made to others;
2. Both parties agree that the children will be allowed to participate in any religious activity or celebration with each parent and their respective families as long as such activity doess not unduly place the children at risk.
3. Both parties will be involved in all medical decisions of the children regardless of who may be caring for the children at the time of their needing medical attention.
Sincerely (insert ex-wife's name here)
CC: congregation elders, both lawyers
Ensure your lawyer reviews before sending. We know active witnesses cannot be reasonable. It is their unreasonableness and extremism that is their greatest weakness. The above are very reasonable things that most reasonable people (and lawyers) would agree to. When you have enough documented evidence of the person breaking these agreements, you take THAT to court. It is then their breaking of the agreement that will make them lose custodial chances not their religious beliefs.
By you keeping religion out of it, he cannot ask for the WT lawyers to come in and help. Thus the reason why you cc the elders in his congregation. They have to approve the society getting involved. If they have this 'olive branch' from you, they hopefully will see your hubby is just trying to use them. But it also gives your lawyer extra ammunition should the cong get involved.
I am not a lawyer but have seen this work. If you look under the topic of "Best of" here, look at Child Custody. There have been several excellent threads on this topic.
Best of luck to your daughter in ensuring the best interests of the child are kept the priority.