(from On the Making of Man, 380 A.D.)
The tree, then, from which comes this fruit of mixed knowledge, is among those things which are forbidden; and that fruit is combined of opposite qualities, which has the serpent to commend it, it may be for this reason, that the evil is not exposed in its nakedness, itself appearing in its own proper nature–for wickedness would surely fail of its effect were it not decked with some fair color to entice to the desire of it him whom it deceives–but now the nature of evil is in a manner mixed, keeping destruction like some snare concealed in its depths, and displaying some phantom of good in its exterior.
The beauty of the substance seems good to those who love money: yet “the love of money is the root of all evil” (1 Tim. vi. 10). And who would plunge into the unsavory mud of wantonness, were it not that he whom this bait hurries into passion thinks pleasure a thing fair and acceptable? So, too, other sins keep their destruction hidden, and seem at first sight acceptable, and some deceit makes them earnestly sought after by unwary men instead of what is good. 
Now since the majority of men judge the good to lie in that which gratifies the senses, and there is a certain identity of name between that which is, and that which appears to be “good,”–for this reason that desire which arises toward what is evil, as though towards good, is called by Scripture “the knowledge of good and evil;” ‘knowledge,’ as we have said, expressing a certain mixed disposition. It speaks of the fruit of the forbidden tree not as a thing absolutely evil (because it is decked with good), nor as a thing purely good (because evil is latent in it), but as compounded of both, and declares that the tasting of it brings death to those who touch it; almost proclaiming aloud the doctrine that the very actual good is in its nature simple and uniform, alien from all duplicity or conjunction with its opposite, while evil is many-colored and fairly adorned, being esteemed to be one thing, and revealed by experience as another, the knowledge of which (that is, its reception by experience) is the beginning and antecedent of death and destruction. 
 Gregory’s view is correct. Because the two qualities of the forbidden tree are mixed within us, we know not how to distinguish between the good and the evil. Men think the ways of death are the ways of life. We walk in darkness, and need light to guide our steps. That light is the Gospel. –B.A.S.
 This is why heresies are so dangerous. When men mix truth with lies they produce fruit from the forbidden tree, which works death in those who partake of it. The truth is not to be played with like a toy. For mingling truth with falsehood advances Satan’s interests in the world. Surely, they who handle the Gospel carelessly will be brought to account at the last day.–B.A.S.