Villains explained :

That man in the basement it was Holly's husband. Loki asked the priest how the man got down there and the priest said that he came to him to confess about kidnapping and killing kids over the past 20 something years. He said they had kidnapped and murdered 16 children and he felt no remorse and would continue to do so. The priest lured him back to his house and he locked him in the basement to die.

What you were expecting to be a cult was only between Holly and her husband and it was prevalent theme throughout the movie.
War With God: I think we can all agree that they lived in a fairly religious town and Holly and her husband were probably pretty religious until their child died from cancer. She then became so angry with God that he took their child away from him that she went crazy and started kidnapping all of these kids to try and make their parents feel the same way her and her husband did. She wanted everyone to be as angry at God as she was.

Bob Taylor was abducted by Holly and her husband as a child (probably not too long after they abducted Alex/Barry). While the police were investigating Taylor's house, they found a book of mazes called "The Invisible Man." Holly's husband was obsessed with this book and the mazes in it and they gave this book to all of the kidnapped children and told them that if they solved all of the mazes, they could go home. The only catch, the last maze was unsolvable. They police clearly stated that during Taylor's captivity he was drugged up so much that by the time he escaped and was rescued, he couldn't remember anything about his captives except for the mazes, leading him to believe that he had be abducted by "The Invisible Man". Taylor then became a PRISONER in his mind and went on through life believing that if he could just solve the last maze he would be free again. Even though he was physically free, he was forever tormented by not being able to solve the maze and go home. Taylor went through the homes taking things as a sort of way to try and remember his own kidnapping, the police referred to him as a "copycat". He was also tormented by the fact that he couldn't remember anything about his own abduction.