Koresh, 33, claimed he had the right to impregnate young women as part of his religious function, Professor Albert Waite told the inquest in Manchester. He said he heard the claims after infiltrating a meeting of the cult outside the Seventh Day Adventists' Newbold College in Bracknell, Berkshire, where he was principal lecturer.
Two of the Britons who died at Waco were students at the college, and the professor said he also knew some of the other victims.
Koresh had "set himself up as the sinful Messiah" and said "he himself would fulfil the prophecy of the second coming of Christ", Professor Waite told the inquest. He advocated multiple sex partners, at least for himself.
He told how Koresh had used a man called Stephen Schneider to recruit theology students from the Seventh Day Adventist Church, from which Koresh had been expelled in 1981.
Schneider, a former student expelled for being drunk and disorderly, had posed as a Seventh Day Adventist and arrived at the college carrying a Bible.He was a "wolf in sheep's clothing", the professor said.
His request for a room in which to hold Bible classes was refused after warnings about Koresh from church elders, but he held meetings in a house outside college. Koresh himself was said to have visited the house.
One of the victims of the Waco siege and fire was a student called John McBean, 27, from Preston, Lancashire. He had tried to get the professor to go to the US to hear things that would "change his life".
"He got upset when I didn't accept Koresh had the right to impregnate young ladies as part of his religious function," he said.
He said Koresh and his followers had targeted the Seventh Day Adventists - from whom the Davidian sect split in the 1930s - and used psychological ploys to lure recruits.
"I beieve Koresh and Schneider led the 23 victims on a road only the very strong could get off, and only if they were allowed to," the professor added.
Earlier, Gale Monbelly told the hearing she was convinced her sister Alison, 31, who died in the compound, was being held against her will. She said she discovered Koresh had been holding her sister's money and passport and that she was not allowed out of the compound. Alison, from London, had joined the Seventh Day Adventists and began attending Bible study groups with other people who were to become victims. Despite being warned by a man who had been to Waco that Koresh was "dangerous and manipulative" she went there in September 1991.
When she returned, she explained the origin of Koresh's assumed name. She said David was from King David in the Bible and that Koresh represented the last sound a person made when life left their body.
She told her that Koresh smoked and drank alcohol and played electric guitar in a band.
"She said from his appearance you would not think he was a religious leader," Ms Monbelly told the inquest. "You would have to listen to what he said to know he was a religious man." In March of the following year, her sister was told by Koresh to returnto Waco. "When they got the call to go back they were told a big event was to happen and they were to get out there as quickly as possible."
Ms Monbelly said: "There is no doubt in my mind that when she went back for the last time she was being held against her will. I think she was very scared. She told me she wasn't allowed out."
She said her sister told her that children in the compound were disciplined and well-behaved. They were not allowed Coca-Cola and sweets.
Ms Monbelly cried as she told the inquest: "I have lost my best friend and closest sister and it is very hard to start the day knowing I will never talk to her, see her, laugh with her or just basically love her."
The inquest continues today.