Jurors at Woolwich Crown Court also found Timothy Storey, 35, guilty of one assault by penetration.
Timothy Storey, formerly of Peckham Grove in Peckham, south London, was working for the Diocese of London when he began grooming two underage girls for sex via telephone calls, social media and texts.
Mr Storey was previously convicted of grooming girls aged 10 to 16 and encouraging them to perform sexual acts via social media in 2014.
The two victims in this current case came forward after hearing of Mr Storey's previous conviction.
Detective Constable Paul Hill, of the Metropolitan Police's Sexual Offences Exploitation and Child Abuse Command, said: "The victims in this case were young teenagers when Storey assaulted them, having been groomed from a very young age.
"In his position as a youth leader he manipulated the victims and forced them to engage in sexual acts that they did not want to do.
"They have shown courage in coming forward after so many years to tell us what happened to them.
"This conviction goes to show that the sexual offences, exploitation and child abuse command will pursue sex offenders and bring them to justice.
"Storey thought he had got away with it, but the victims in this case were determined to see him brought to justice."
A spokeswoman for the Diocese of London said: "Timothy Storey has today been convicted of a series of appalling crimes and we are profoundly sorry for what his victims endured."
She also said the Diocese first received complaints about Storey in early 2009 and spoke with police and withdrew him from ordination training.
