How did this high ranking Mormon lose his faith? It wasn't the result of someone getting in his face and telling him he was an idiot. The NYT tells the story:

When fellow believers in Sweden first began coming to him with information from the Internet that contradicted the church’s history and teachings, he dismissed it as “anti-Mormon propaganda,” the whisperings of Lucifer. He asked his superiors for help in responding to the members’ doubts, and when they seemed to only sidestep the questions, Mr. Mattsson began his own investigation.

But when he discovered credible evidence that the church’s founder, Joseph Smith, was a polygamist and that the Book of Mormon and other scriptures were rife with historical anomalies, Mr. Mattsson said he felt that the foundation on which he had built his life began to crumble.