Mother Theresa, probably one of the most famous Christians alive was an agnostic/atheist during most of her life. Below are a few citations from private letters that she wrote:
“So many unanswered questions live within me afraid to uncover them … because of the blasphemy … If there be God … please forgive me … When I try to raise my thoughts to Heaven there is such convicting emptiness that those very thoughts return like sharp knives & hurt my very soul. I am told God loves me … and yet the reality of darkness & coldness & emptiness is so great that nothing touches my soul.”
“What do I labor for?” she asked in one letter. “If there be no God, there can be no soul. If there be no soul then, Jesus, You also are not true.” (1)