More than 20 years ago, the Rev. Thomas P. Doyle, a lawyer assigned to the Vatican Embassy, tried to warn the Vatican that it was facing a potential scandal involving children and allegations of sexual abuse by priests.

Since then, the priest has worked with 2,000 victims of clerical sexual abuse and testified on behalf of victims in 200 court cases.

A new book co-written by Doyle, "Sex, Priests and Secret Codes: The Catholic Church's 2,000-Year Paper Trail of Sexual Abuse," makes a case that sexual abuse has been a problem in the Catholic Church since its earliest days — even the fourth century.

Papal decrees were issued in an attempt to regulate the sex lives of the clergy, the authors say. "In days when priests were allowed to marry, we find laws telling them to avoid sex; when celibacy became mandatory for the clergy, we find laws against concubinage. We also find condemnations of homosexuality in the ranks of the clergy; the sexual abuse of minors; and the solicitation of sex by priests in the confessional."

"I knew it went back decades; I just didn't know how far," said Doyle, who recently retired as a U.S. Air Force chaplain. "There is evidence that the bishops tried to stop these things and discipline priests, but there is (little) mention of any discipline against those who cover it up."

To write the book, Doyle joined with Patrick Wall, a former Benedictine monk who has investigated hundreds of sexual-abuse claims against priests, and Richard Sipe, a former Benedictine monk who became a psychotherapist specializing in treating mental health issues of priests and bishops for almost 30 years.

"Sex, Priests and Secret Codes" is the first of three books that the three plan to do together.

In modern times, Sipe said, there are cases of sexual abuse of minors dating from the 1920s. "In reviewing depositions with bishops, they said over and over again, 'We knew nothing,' but the truth is that it was officially ignored," said Sipe. Documents from 12 different grand jury investigations indicate it was well-known, he said. "It's undeniable."

Bill Ryan, a spokesman for the U.S. Conference of Catholic Bishops, says he takes issue with the book's premise that there has been long-standing knowledge of sexual abuse by priests.

"I don't think anybody in the church knew the extent of this until recent times," Ryan said. "This is something that also exists in society, and most people didn't know or talk about these kinds of things until recently."

Ryan said the Catholic Church has taken steps to weed out priests with sexual problems and prevent them from entering the seminary.