My 2 cent prognostication,
Well I would ask your ex-wife what she thought would happen if the court found out that she was using her visitation rights primarily for religious instruction in a faith contrary to the child's upbringing? This is based on the information that has already been related by your grandchild that this was the case.
Why don't you suggest keeping this out of the courts and giving your ex supervised visitation to start with, and if she plays ball and when a trust relationship has been established over some months, she might be allowed to re-establish sleep overs etc. on the proviso that faith based discussions are off limits. If the trust is violated after this agreement she then will have to go to court.
If she really loves your grandchild enough she would probably agree to the idea.