And he was LDS.  Check out this Wikipedia article on Kim Peek.

Here’s the key paragraph:

In 1984, script writer Barry Morrow met Peek in Arlington, Texas; the result of the meeting was the 1988 movie Rain Man. The character of Raymond Babbitt, although inspired by Peek, was portrayed as having autism. Dustin Hoffman, who played Babbitt, met Peek and other savants to get an understanding of their nature and to play the role with accuracy (see Method acting). The movie caused a number of requests for appearances, which increased Peek’s self-confidence. Barry Morrow gave Kim his Oscar statuette to carry with him and show at these appearances. It has been referred to as the “Most Loved Oscar Statue” since it’s been held by more people than any other Oscar statue. Kim also enjoyed approaching strangers and showing them his talent for calendar calculations by telling them on which day of the week they were born and what news items were on the front page of major newspapers. Peek had also appeared on television. He traveled with his father, who took care of him and performed many motor tasks that Peek found difficult.^[4]

Kim Peek, RIP.