[rmlew]

If their maternal grandmother was Jewish, then they would be Jewish, irrespective of the ethnicity or religious orientation of the other three grandparents.

If the mother is raised as a Christian, the child is a christian.

[Lessismore]

Probably not if she was raised Christian, e.g. raised in a Catholic orphanage from an early age.

However, if she was raised Jewish, then divorced a Jewish husband without a Jewish divorce, converted and married a Christian husband, and you are the offspring of the latter union -- then you've got a problem.