High Priestess and the sinners

"So cut the rose in full bloom, 'till the fearless come and the act is done..."

When the Priestess descended to Earth and justice was done, she chose two dead sinners and sent them one more time to the world of alive. She did it for a reason, but for certain purposes. She will give them another life, exchange for the lives of two girls.

What are the goals of the Priestess and why did she need to change the fate of two completely different young girls living at different times, with the help of two young dead fellows, who ceased their lives at different times for ridiculous reasons?