What Happens if a Priest Has a Mortal Sin and Serves Liturgy

Bishop Palladius. "The Lausiac History".

Chapter XVIII.  Macarius of Alexandria

Once I visited this holy Macarius and found a village priest lying just outside his cell, whose head was all eaten away by the disease called cancer, and the actual bone appeared on the crown of his head. He had come to be healed and Macarius would not grant him an interview. So I besought him: "I pray you, pity him and give him his answer." And he said to me: "He does not deserve to be healed, for it has been sent him as a punishment. But if you want him to be healed, persuade him to give up taking services. For he was taking services, though living in fornication, and for this reason he is being punished and God is healing his soul." So when I said this to the afflicted man he consented, and swore that he would no longer exercise his priesthood. Then he received him and said: "Do you believe that God is?" He said to him: "Yes." "Were you able to mock God?" "No," he answered. He said: "If you recognize your sin and the chastening of God, on account of which you suffered this, reform yourself henceforward." So he confessed his fault and gave a promise that he would sin no more nor take the service, but embrace the position of a layman. Then he laid his hands on him and in a few days he was cured and the hair grew and he went away healed.