Thomas P. Doyle is a Dominican priest with a bachelor's and five master's degrees, a canonical lawyer with a doctorate in canon law and a veteran expert witness on Catholic theology and the church.

He has also worked to understand the institutional and moral failures behind the abuse of minors by priests since 1984, when the scandal first erupted in the United States in Louisiana.

Doyle appeared in Philadelphia Common Pleas Court last Thursday at the trial of two Catholic priests involving the sexual abuse of children by some priests in the Archdiocese of Philadelphia. Assistant District Attorney Patrick Blessington wanted Doyle to explain to the jury some of the more esoteric elements of Catholic theology.

Doyle was parsing the Seven Sacraments of the Catholic Church and discussing the significance of baptism, the ceremony in which the priest welcomes an infant into the church and washes away "original sin" – Adam and Eve's sin of disobedience to God in the Garden of Eden – that Catholics believe all people are born with.

That brought Blessington to the concept of "Limbo," the name in traditional Catholic theology for the afterlife of infants who are not baptized and who die in a state of "original sin." It's a netherworld without the promise of Heaven or the punishment of Hell. It's long been a problematic concept for many Catholics and in 2007 the church's International Theological Commission suggested that Limbo was one theory about the possible fate of unbaptized infants but did not have the theological surety of Heaven and Hell.

What's a witness to do? For all his learning, Doyle decided to avoid the arcane and, to the laughter of almost everyone in the courtroom, define Limbo in lay terms.

"The best way I can describe it is minimum-security Hell," Doyle said.