The heretics of our time claim that there are two principles of things: the principle of light, and the principle of darkness.
They say the principle of light is God, from whom come spiritual things, namely souls and angels; the principle of darkness is Lucifer, from whom come temporal things. This they strive to prove by authorities and by reasonings. By authorities, thus: A good tree cannot bear evil fruit, nor an evil tree good fruit (Matth. 7). Therefore since God is supremely good, evil things do not come from him; but since evil things exist, and do not come from God, therefore they come from someone other than God. Therefore, since God is the principle of good things, the principle of evil things is something else. Likewise, in the beginning of Genesis it is read that darkness was upon the face of the deep (Gen. 1), and thus the world had its beginning from darkness. And so the creator of the world was the principle of darkness, and thus the author of the world was evil, since the beginning of his creation began from darkness. Likewise Christ says in the Gospel: The prince of this world comes, and he has nothing in me (John 14). There he calls Lucifer the prince of the world rather than himself, and so Lucifer was rather the author of the world than Christ. And elsewhere it is said: No one can serve two masters (Matth. 6), namely God and mammon. There Christ calls himself and mammon, that is the devil, "masters"; but Christ is not called Lord except by reason of creation; therefore the devil too, is called lord by a similar reason of creation. Likewise: You are of your father the devil (John 8). Likewise the Apostle: It is not I who do it, but sin which dwells in me, that is, in my flesh (Rom. 7). If sin dwells in the flesh, and comes from the flesh, and flesh cannot be without sin, then the flesh is evil, and thus does not come from God. Likewise: I perceive another law in my members, warring against the law of my mind, and taking me captive in the law of sin, which is in my members (ibid.).
If the law of the flesh is the law of sin and death, then flesh seems to be evil, and therefore does not come from God. Likewise the same author says: The flesh lusts against the spirit, and the spirit against the flesh (Gal. 5). If flesh always contradicts reason, flesh is evil, and thus comes from evil.