Williams, a moral theologian, author, lecturer and U.S. television personality, admitted last year that he had fathered a child several years earlier.
At the time, Williams apologized for "this grave transgression" against his vows of celibacy and said he had stayed on as a priest because he hoped to move beyond "this sin in my past" to do good work for the church. The Legion's retired superior later admitted he had learned about the child in 2005 but allowed Williams to keep teaching and preaching about morality.
After taking a year off for reflection, Williams left the priesthood in May to care for his son. According to their wedding registry, he and Lev are due to marry on Saturday in the United States.
Asked for comment Thursday, Lev confirmed the wedding plans in an email, adding: "We have no intention of ever discussing our personal life in this forum."
She had initially denied an intimate relationship with Williams, though they frequently appeared together in American circles in Rome, particularly with visiting U.S. student and Catholic tour groups.
Their wedding closes a circle of sorts, even as it raises some uncomfortable questions: Who beyond Williams' superior in the church knew about the child while the couple tried to cover it up? Was Williams already in a relationship with Lev when she was hired at the magazine he published? And did the family ties to Williams influence Glendon in her defense of the Legion and its disgraced founder despite credible reports that the founder was a pedophile?
The saga of the Legion of Christ represents one of the most egregious examples of how the Vatican ignored decades of reports about sexually abusive priests as church leaders put the interests of the institution above those of the victims.
The Rev. Marcial Maciel founded the cult-like Legion in 1941 in Mexico and oversaw its growth into a large and prominent congregation despite credible reports that he was a drug addict and child molester. After Maciel's death in 2008, the Legion admitted that he fathered three children and sexually abused his seminarians.
In 2010, the Vatican took over the order and a papal delegate has been overseeing a reform and "purification." In January, the Legion will elect a new leadership and approve a new set of constitutions.
The Legion scandal has been particularly damaging to the Vatican because Maciel was held up by Pope John Paul II and his cardinals as a model for the faithful, with the order admired for its orthodoxy and ability to bring in money and new priests.
Like all Legion priests, Williams had been a staunch defender of Maciel. When Maciel's double life became public in 2009, Williams told the Catholic ETWN program that the revelations were a "very, very hard blow to all of us."
Until he left active ministry, Williams was the most publicly prominent priest in the 900-strong order. He is the author of such books as 2008's "Knowing Right From Wrong: A Christian Guide to Conscience," and was a commentator for the U.S. broadcaster CBS. He was the superior of the Legion's general directorate in Rome in the late 1990s and early 2000s.