Pastor Mark Pitcher of Porterville Church of the Nazarene tells TMZ ... ordinarily he'd have a massive conflict performing a funeral service for someone with Manson's homicidal past, but he made an exception because Manson's grandson, Jason Freeman, is a Christian who was in need of a pastor.
Pitcher tells us the funeral director approached him last week with what he called a "unique situation." He says he agreed to meet Jason and his wife, Audrey, the day before the service.
Pitcher says he told Jason he wouldn't touch certain aspects of Manson's life -- namely the Manson Family murders. He did read a few scriptures, at Jason's request, during the ceremony.
TMZ broke the story ... Manson had an open casket funeral over the weekend. He was immediately cremated when the service ended and his ashes spread along a nearby creek bed.