5 And may it be that we say this over-hastily? Let the apostle himself make this plain in what follows. Remember what was said at the outset of our discourse, that the present epistle commends charity: “God is light,” says he, “and in Him is no dark ness at all.” And what said he above? “That ye may have fellowship with us, and our fellowship may be with God the Father, and with His Son Jesus Christ.” But moreover, if “God be light, and in Him is no darkness at all, and we must have fellowship with Him,” then from us also must the darkness be driven away, that there may be light created in us, for darkness cannot have fellowship with light.
To this end, see what follows: “If we say that we have fellowship with Him, and walk in darkness, we lie.” You have also the Apostle Paul saying, “Or what fellowship has light with darkness?” You say you have fellowship with God, and you walk in darkness; “and God is light, and in Him is no darkness at all:” then how should there be fellowship between light and darkness? At this point therefore a man may say to himself, What shall I do? How shall I be light? I live in sins and iniquities.
There steals upon him, as it were, a desperation and sadness. There is no salvation save in the fellowship of God. “God is light, and in Him is no darkness at all.” But sins are darkness, as the Apostle says of the devil and his angels, that they are “rulers of this darkness.” He would not call them of darkness, save as rulers of sins, having lordship over the wicked. Then what are we to do, my brethren? Fellowship with God must be had, other hope of life eternal is none; now “God is Light, and in Him is no darkness at all:” now iniquities are darkness; by iniquities we are pressed down, that we cannot have fellowship with God: what hope have we then?
Did I not promise to speak something during these days, that shall cause gladness? Which if I make not good, this is sadness. “God is Light, and in Him is no darkness at all;” sins are darkness: what shall become of us? Let us hear, whether perhaps He will console, lift up, give hope, that we faint not by the way. For we are running, and running to our own country; and if we despair of attaining, by that very despair we fail. But He whose will it is that we attain, that He may keep us safe in our own land, feeds us in the way.
Hear we then: “If we say that we have fellowship with Him and walk in darkness, we lie, and do not the truth.” Let us not say that we have fellowship with Him, if we walk in darkness. “If we walk in the light, as He 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 are we to do about our sins? Hear what follows, “And the blood of Jesus Christ His Son shall purge us from all sin.” Great assurance has God given!
Well may we celebrate the Passover, wherein was shed the blood of the Lord, by which we are cleansed “from all sin!” Let us be assured: the “handwriting which was against us,” the bond of our slavery, the devil held, but by the blood of Christ it is blotted out. “The blood,” says he, “of His Son shall purge us from all sin.” What means, “from all sin”? Mark: lo even now, in the name of Christ whom these here have now confessed, who are called infants, have all their sins been cleansed.
They came in old, they went out new. How, came in old, went out new? Old men they came in, infants they went out. For the old life is old age with all its dotage, but the new life is the infancy of regeneration. But what are we to do? The past sins are pardoned, not only to these but to us; and after the pardon and abolition of all sins, by living in this world in the midst of temptations, some haply have been contracted. Therefore what he can, let man do; let him confess himself to be what he is, that he may be cured by Him who always is what He is: for He always was and is; we were not and are.
Source: Homilies on the First Epistle of John (New Advent)