Kennedy Ife's parents and brothers mistakenly believed he was "possessed by evil spirits" and set about "curing" him with prayer and restraint, jurors were told.
They allegedly used cable ties, rope and handcuffs to restrain the "vulnerable" 26-year-old at home in Enfield, north London, in the days before his death in August 2016.
Kennedy's parents, Kenneth and Josephine, and five brothers Roy, Harry, Colin, Samuel and Daniel are on trial at the Old Bailey accused of his manslaughter.
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Prosecutor Tom Little QC said: "This is a case in which a young man in his 20s lost his life. The reason is that his own family decided that because he had become and remained unwell he should be restrained with cable ties, rope and handcuffs.
"They decided that he should be restrained in his own home with them rather than calling 999. Ultimately that restraint was a cause of his tragic death.
"The 999 call that was eventually made for expert assistance was too late.
"The family put misguided religious beliefs about this young man being possessed by a demon or evil spirits before the obvious and common sense conclusion that he needed help and did not need to be restrained."
Mr Little said the Ife family held deep and unusual religious beliefs, described by one of the defendants as "charismatic Christians".
When Kennedy showed signs of illness in August 2016, they allegedly believed he had become possessed by evil spirits and decided to restrain him, jurors heard.
Mr Little told jurors: "Instead of calling 999 or seeking other expert assistance they sought to 'cure' his perceived illness through prayer and restraint.
"Whether this was some form of exorcism will be a matter for you in due course.
"In short, the Ife family took the law into their own hands."
Josephine Ife, 56, and Kenneth Ife 64, deny manslaughter, false imprisonment and causing or allowing the death of a vulnerable adult between August 19 and 23 2016 along with their sons Colin, 26, twins Daniel and Samuel, 20, Harry, 32, and Roy 33.