Timothy Storey, 35, preyed on his victims at religious summer camps he worked at which they attended "from an early age", befriending them over time.
Mr Storey groomed and sent them sexual texts and messages on social media, and eventually manipulated both women into meeting with him.
Neither of the women can be named for legal reasons.
In the first case, Storey, formerly of Peckham Grove in Peckham, south London, subjected one of the girls to inappropriate and sexual phone calls.
He used his position as a youth leader to gain the victim's trust, and sexually assaulted her on two separate occasions between 2008 and 2009.
In the other case, the teenager agreed to go to a concert with Storey, and afterwards he assaulted her at his home.
He was found guilty of three counts of rape and one assault by penetration in February following a trial at London's Woolwich Crown Court.
The 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.
Mr Storey is due to be sentenced at Woolwich Crown Court on Friday.
