Saint Ambrose tells us that we have to humble ourselves before God, confessing that wickedness comes not from without but from within ourselves. He is, in effect, repeating the teaching of Our Lord: “There is nothing from without a man that entering into him, can defile him. But the things which come from a man, those are they that defile a man” (Mark 7:15).
At the same time, Saint Ambrose gives hope to the feeble and infirm, alluding to the words of Our Lord to Saint Paul, “My grace is sufficient for thee; for power is made perfect in infirmity” (2 Corinthians 12:9).
From the Treatise on the Six Days of Creation by St Ambrose, Bishop & Doctor
If evil is not something that has no principle from which it springs, as is the case of the uncreated Being; and if it is not something that God has made, where does it come from? For no wise person will deny the existence of evil in this world. We are all familiar with the evil of death. But from what we have said it is evident that evil is not a living substance.