John Laister was convicted at Bradford Crown Court of three counts of rape, four counts of indecent assault and four counts of indecency with a child.
Some of the offences were committed more than 40 years ago when he worked at a children's care home in Bramhope, Leeds, and at a church in Bradford.
Judge Benson said Laister was "a sexual predator of monstrous proportions".
Laister, 72, of Sunningwell, Abingdon, Oxfordshire, previously admitted four counts of indecent assault and will be added to the sex offenders register for the rest of his life.
He was a former "house parent" at the now-closed Hilton Grange home when he raped a vulnerable girl under his care.
After being moved on following allegations against him, he continued to abuse young girls while acting as a Sunday school teacher at the Thorpe Edge Evangelical Church.
The court heard one of the victims, who was raped and abused at the home, had suffered an epileptic fit after giving evidence via a videolink.
The judge said "the main object of his loathsome activities" was a victim who found the courage to come forward in 2012.
Det Con Naomi Brown, of West Yorkshire Police, said: "Laister took advantage of his victims' young ages and vulnerabilities to carry out these attacks over a period of time.
"We would like to thank them for reporting these offences to the police, and we hope that the lengthy sentence passed down today will help to give them some closure."
An ex-Sunday school teacher who raped and sexually abused young girls under his care has been jailed for 14 years.