PHILADELPHIA — A Roman Catholic church official will remain in prison while he fights his landmark conviction for failing to protect a boy from a predator priest.

A judge deemed Monsignor William Lynn’s conduct too serious on Monday to warrant bail, despite defense arguments that the felony conviction may well be overturned because Lynn wasn’t the immediate supervisor of any accused priests.

‘‘This is the first case in the history of the Commonwealth where [endangering the welfare of a child] has been applied to a supervisor on Monsignor Lynn’s level,’’ defense lawyer Jeffrey Lindy said.

Lynn, the longtime secretary for clergy, is serving a three- to six-year prison term. He was the first US church supervisor convicted of endangering children by helping the church shuttle accused priests from church to church.

Common Pleas Judge M. Teresa Sarmina, in sentencing Lynn last month, said he had ‘‘enabled monsters in clerical garb . . . to destroy the souls of children.’’

Defense lawyers were not surprised by her bail decision and planned an immediate notice of appeal to move the case to state Superior Court.

But Lynn’s legal team is shrinking as the Philadelphia archdiocese caps its budget for his appeal. Lindy is stepping down after eight years representing Lynn, while two of the four lawyers who defended him at trial will continue ‘‘largely on a pro bono basis,’’ the archdiocese said.

Lynn’s lawyers are ‘‘strongly convinced that there were many errors at trial and the sentence is disproportionate to other punishments meted out to administrators for this same charge,’’ the archdiocese said in a statement. ‘‘We hope that the ultimate decision in Monsignor Lynn’s regard is just and merciful.’’

The church spent millions on defense lawyers during an extraordinary 10-year criminal investigation. The late Cardinal Anthony Bevilacqua, Lynn, and other church leaders testified repeatedly — with lawyers at their side — before the first of two grand juries. Prosecutors concluded that at least 63 predator-priests had worked in Philadelphia, often for decades.

Lynn, now 61, was the only church official charged.

‘‘I think the Commonwealth demonized him,’’ Lindy said. ‘‘He is serving somebody else’s time. That is an injustice.’’