TEHUACÁN, Mexico — For two decades, dozens of children have accused the Rev. Nicolás Aguilar of molesting or brutally raping them. He faces an indictment charging sexual abuse in Los Angeles and at least five formal complaints in Mexico. Yet at 65 he remains at large, still working as a priest in villages here. In the Tehuacán area, there were complaints about Father Aguilar. Father Aguilar’s long flight from the law, critics say, reflects the ease with which priests can avoid prosecution in the United States by hiding in Mexico, where judges and prosecutors are reluctant to challenge the enduring...