Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Donatist Controversy.: Chapter 54

Chapter 54.—123.  Petilianus said:  "But if it should so happen, though whether it be so I cannot say, that you cast out devils, neither will this in you do any good; because the devils themselves yield neither to your faith nor to your merits, but are driven out in the name of the Lord Jesus Christ."

124.  Augustin answered:  God be thanked that you have at length confessed that the invocation of the name of Christ may be of profit for the salvation of others, even though it be invoked by sinners!  Hence, therefore, you may understand that when the name of Christ is invoked, the sins of one man do not stand in the way of the salvation of another.  But to determine in what manner we invoke the name of Christ, we require not your judgment, but the judgment of Christ Himself who is invoked by us; for He alone can know in what spirit He is invoked.  Yet from His own words we are assured that He is invoked to their salvation by all nations, who are blessed in the seed of Abraham.