How do Proctor, Francis Nurse, and Giles Corey plan to use Mary's testimony to prove that "heaven is NOT speak through the children?"

Basically Mary agrees, under duress, to tell the truth. In her deposition, Mary says the whole witchcraft thing is a sham masterminded by a vindictive Abigail. The girls were doing forbidden things in the forest for fun. This proves that children aren't as holy and innocent as the Church would like to believe.