Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Donatist Controversy.: Chapter 40

Chapter 40.—95.  Petilianus said:  "Paul the apostle also bids us, ‘Be ye not unequally yoked with unbelievers:  for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? and what concord hath Christ with Belial? or what part hath he that believeth with an infidel?’" 2115

96.  Augustin answered:  I recognize the words of the apostle; but how they can help you I cannot see at all.  For which of us says that there is any fellowship between righteousness and unrighteousness, even though the righteous and the unrighteous, as in the case of Judas and Peter, should be alike partakers of the sacraments?  For from one and the same holy thing Judas received judgment to himself and Peter salvation, just as you received the sacrament with Optatus, and, if you were unlike him, were not therefore partakers in his robberies.  Or is robbery not unrighteousness?  Who would be mad enough to assert that?  What fellowship was there, then, on the part of your righteousness with his unrighteousness, when you approached together to the same altar?

Footnotes

2 Cor. 6:14, 15.