A Florida couple have been arrested for allegedly turning a 13-year-old girl into their sex slave and sealing her off from the outside world for more than five years.

Rob Johnson, 44, and his wife, Marie Johnson, 43, from Port St. Lucie, took in the then 13-year-old girl to live with them after her mother died.

Soon after her arrival the teenager was told that she could only be part of the family if she had sex with the couple, according to the Port St. Lucie Police Department.

The pair were arrested on Tuesday and charged with felony sexual assault charges.

The girl told investigators that she was forced to call Rob “master,” and that Marie Johnson had pushed her against a wall and held her by her throat until she agreed to the meet the sexual demands.

She also said that she was beaten when she failed to follow the couple’s directions or complete her chores.

The girl, who has not been identified, said that the abuse happened repeatedly for five years until she was 18 years old.

According to police, throughout this time Rob Johnson held Sunday school classes for his family at their home, invoking Old Testament Bible Passages to justify the couple's actions.

Police said that the girl, now aged 21, was home schooled, not allowed to use the phone and forced to rehearse what to say to doctors if she was ever asked about her sexual activity.

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The girl was able to leave the home when her grandmother bought her a plane ticket to bring her to Ohio, police said.

The Johnsons were freed on bail after appearing at St Lucie County Court on Tuesday.