1 John 1:6

Ex iis autem, qui ab hæresi ducuntur, Marciohis quidem Pontici fecimus mentionem, qui propter certamen, quod adversus Creatorem suscepit, mundanarum rerum usum recusat. Ei autem continentiæ causa est, si modo est ea dicenda continentia, ipse Creator, cui se adversari existimans gigas iste cum Deo pugnans, est invitus continens, dum in creationem et Dei opus invehitur. Quod si usurpent vocem Domini, qui dicit Philippo: “Sine mortuos sepelire mortuos suos, tu autem sequere me:” at illud considerent, quod similem cam is formationem fert quoque Philippus, non habens cadaver pollutum. Quomodo ergo cum carhem haberet, non habuit cadaver? Quoniam surrexit ex monumento, Domino ejus vitia morte afficiente, vixit autem Christo. Meminimus autem nefariæ quoque ex Carpocratis sententia mulierum communionis. …

From the Epistle also of John they forthwith cull (a proof). It is said: “The blood of His Son purifieth us utterly from every sin.” Always then, and in every form, we will sin, if always and from every sin He utterly purifies us; or else, if not always , not again after believing; and if not from sin, not again from fornication. But what is the point whence (John) has started? He had predicated “God” to be “Light,” and that “darkness is not in Him,” and that “we lie if we say that we have communion with Him, and walk in darkness.” “If, however,” he says, “we walk in the light, we shall have communion with Him, and the blood of Jesus Christ our Lord purifieth us utterly from every sin.” Walking, then, in the light, do we sin? and, sinning in the light, shall we be utterly purified? By no means. For he who sins is not in the light, but in darkness. …

“And the light shineth in darkness and the darkness hath not overtaken it.” We are still enquiring about the light of men, since it is what was spoken of in the preceding verse, and also, I consider, about darkness, which is named as its adversary, the darkness also being, if the definition of it is correct, that of men. The light of men is a generic notion covering two special things; and with the darkness of men it is the same. He who has gained the light of men and shares its beams will do the work of light and know in the higher sense, being illuminated by the light of the higher knowledge. And we must recognize the analogous case of those on the other side, and of their evil actions, and of that which is thought to be but is not really knowledge, since those who exercise it have the reason (Logos) not of light but of darkness. …

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 commendeth charity: “God is light,” saith 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. …