A picture taken on November 23, 2014 shows the archbishop of Granada Francisco Javier Martinez (3-R) prostrating himself on the floor at the city's cathedral during a mass in a gesture of apology to victims of abuse ( AFP/File )
Their accuser, now aged 25, says he was raped and made to perform sex acts with priests at a villa with a swimming pool, according to a written ruling by the judge in the southern city of Granada.
It is the latest in a series of pedophilia scandals involving members of the Catholic Church, and the biggest such legal case known so far in Spain.
Judge Antonio Moreno formally accused 10 priests and two lay people of sexually abusing the boy or being complicit in such acts from 2004 to 2007.
The victim alleges "continuous sexual abuse by a group of priests of the diocese of Granada, from 2004 when he was 14 to 2007 when he was 17."
The young man said he became an altar boy at the age of seven at the church presided by the leader of the group and was later invited to houses run by the priests.
The lead priest made the boy give him massages, which led to a series of acts of sexual abuse, the ruling said.
The victim said the group's leader told him he had a promising career as a priest ahead of him and chided him for resisting, according to the ruling. It said the priest described himself as being like a father to the boy.
The young man, identified in the ruling as a member of the Catholic institution Opus Dei, now suffers anxiety and is undergoing psychological treatment, it said.
It is the most serious such case ever in Spain "in terms of the numbers of abusers and the system of abuse," said Jose Manuel Vidal, head of religious news website Religion Digital, which first reported the affair.
"They set up a kind of sect within the church itself."
The case was opened in early November and the judge had ordered investigations to be kept secret until he issued the ruling indicting the suspects. He has yet to rule whether some of the alleged deeds happened too long ago to be tried.
Pope Francis revealed after the case was launched that he had ordered a Church investigation when the unidentified man wrote a letter telling him of the abuse.
"I called the person and I told him, 'Go to the bishop tomorrow,' and then I wrote to the bishop and told him to start an investigation," Francis told reporters on November 25.
The Argentine pontiff said the case cause him "very great pain, but the truth is the truth and we should not hide it."
At least one other person now aged 44 has also accused one of the priests of abuse in the early 1990s.
The Archdiocese of Granada said in mid-November that it had suspended a number of priests pending the court probe.
The Archbishop of Granada, Francisco Javier Martinez and several other priests prostrated themselves on the floor of the city's cathedral during a mass in November in a gesture of apology to victims of abuse.
Pope Francis has taken a tough stance on clerical child abuse since taking over in 2013 from Benedict XVI.
But a victims' support group, the Survivors Network of those Abused by Priests, said the pope should be tougher.
"The judge in the case has made it clear that covering up child rape is a despicable crime," it said in a statement on Wednesday reacting to the Granada ruling.
"Pope Francis should send the message that this behavior is not going to be tolerated in his Church by immediately firing Granada's Archbishop."