From the Peninsula to Maryland: Costello's role in the summer of 1862
Father Michael Costello became the pastor of St. Peter's Catholic Church in Harpers Ferry, Virginia in 1857. He wrote a firsthand account, in a letter to fellow priest Father Harrington, of John Brown's raid in 1859. Costello also was the only clergymen to remain in Harpers Ferry during the Civil War. Dr. Nicholas Marmion, the only doctor to remain in Harpers Ferry during the war, and Father Costello stayed in Harpers Ferry to aid the remaining citizens.
Costello is noted for flying a Union Jack flag atop of St. Peter's to express neutrality. This action is attributed with saving the church from being a target during the many bombardments the town. He held services and administered sacraments as much as possible during wartime along with allowing the church to be used as a hospital.
Annie Marmion, the Dr. Nicholas Marmion's daughter, would write about the experiences of living in Harpers Ferry during the war. Many of those memories tell of Father Costello and how he protected church property, was a faithful friend to her father, and how he risked his life to be a "doctor of souls."
Costello survived the Civil War and saw his church survive as well. He served as the priest of St. Peter's until he died of illness in 1867.