Tertullian, Against Marcion 2

Ponticus drags out two gods like the twin Symplegades that wrecked his ship: the Creator, our God, whom he cannot deny, and his own deity, whom he will never prove. He suffered this catastrophe, poor wretch, because he let a single, simple passage from the Lord's teaching — the parable of the good and bad tree, applied to human beings, not gods — drive him to the presumption that a good tree cannot bear bad fruit nor a bad tree good fruit; in other words, a good mind or faith cannot produce evil works, nor an evil mind good works. He stumbled, as many do now, especially heretics, over the problem of evil — where does it come from? — and, senses dulled by the sheer bulk of his own curiosity, he latched onto the Creator's declaration: "I am the one who creates evil." The more he had already presumed the Creator to be the author of evil, and the more other arguments that seduce every perverse mind reinforced that view, the more he read the bad tree bearing bad fruit — clearly evil fruit — into the Creator, and concluded there simply had to be another god to serve as the good tree bearing good fruit. Spotting in Christ what looked like an entirely different disposition, one of sheer, unmixed kindness that contrasted with the Creator's, he easily argued a new, alien divinity revealed in his Christ, and with that pinch of leaven he soured the entire batch of faith with heretical bitterness. He also had a certain Cerdo as the instigator of this scandal: two blind men thought they could more easily perceive two gods than one, because they had never seen the one God whole; even to the bleary-eyed, a single lamp looks like many. So he demolished the God he was forced to confess by slandering evil, and built up the god he tried to invent by exalting the good. We will expose the natures he has assigned to these articles through our own responses.