From the book Faith and Life: readings compiled from ancient writers is this selection from St. Augustine:
I. LIGHT IN THE LORD.
IF “God is light, and in Him is no darkness at all,” and if we ought to have fellowship with Him, the darkness must be expelled from us, that light may be kindled in us; for darkness can have no fellowship with light; therefore see what follows, “But if we say that we have fellowship with Him, and walk in darkness, we lie;” and you have the Apostle Paul saying, “What communion hath light with darkness?” Therefore let a man now say to himself, “What shall I do? how shall I become light? I live in sins and iniquities. A feeling of despair and gloom steals over me. There is no salvation except in fellowship with God. God is light, and in Him is no darkness at all. But sins are darkness; iniquities are darkness. We are pressed down by iniquities, so that we cannot have fellowship with God; what hope then have we? what will become of us?” Let us listen, if haply He will comfort and uplift us, and give us hope, lest we faint by the way. For we are hastening to our Country; and if we despair of reaching it, we faint from our very despair. But He who wishes us to reach it, feeds us by the way, that He may preserve us in the Country. “But if we walk in the light, as He also is in the light, we have fellowship one with another.” Let us walk in the light, as He is in the light, that we may be able to have fellowship with Him. And what shall we do about our sins? Hear what follows; “And the Blood of Jesus Christ His Son cleanses us from all sin.” It is great ease of mind that God has given us. The devil once held a bond of slavery against us, but it was obliterated by the Blood of Christ. If you have confessed your sins, truth is in you; for truth itself is light. Your life is not yet perfectly lustrous, for sins are there; but yet you now begin to be illuminated, for confession of sins is there.
–St. Augustine on First Epistle of St. John, Tr. i.