Timothy Storey, 35, met his victims at a church summer camp before bombarding them with sex texts and social media messages to manipulate them into meeting him again.
One was later attacked at his Oxford home after he took her to a concert and plied her with alcohol.
He used his respectable position as a youth leader to gain the second victim's trust, and sexually assaulted her on two separate occasions between 2008 and 2009.
The judge, Philip Katz QC, said his "insidious" behaviour meant he was a serious danger to the public.
He order Storey to serve an extended period of four years on licence on top of his 15-year custodial sentence.
The two women decided to come forward and tell police about their ordeals after Storey was convicted in May 2014 of grooming girls aged 10 to 16 and encouraging them to perform sexual acts via social media.
Storey was found guilty of three counts of rape and one of assault by penetration in February following a retrial at London's
Woolwich Crown Court and was jailed at the same court on Friday.
Storey, formerly of Peckham, south London, showed no emotion as the judge passed sentence.
Judge Katz told him: "You manipulated them into situations where you knew it was going to be their word against yours.
"They both trusted you, as did many others. Your behaviours also demonstrates rank hypocrisy."
Both women were over the age of consent, the judge said, but were still vulnerable and unworldly.
"The psychological harm you caused her was so severe. The incident changed the course of her life," he said.
He added: "There is a streak of arrogance and entitlement in your behaviour."
Discussing the other victim, the judge said: "Your influence over her was total. You had been insidiously grooming her for a year."
This victim wrote a victim impact statement, some of which was read to the court. She said she was glad her story was being heard, adding: "I will be living with these scars for the rest of my life."
Storey's parents sat through both trials and were in court to see him sentenced on Friday.