Was sin necessary for knowledge?

[Belacqua]

Note that they don't just get knowledge of evil through sinning - they get knowledge of both good and evil, even though they hadn't done good.

There's a nicely worked-out view of this that maybe isn't exactly mainstream. It appeals to me, though, and it's the interpretation that Dante uses in his Purgatorio -- so it has a pretty good pedigree. 

First, the idea is that Adam could name the animals even though he didn't have knowledge of them. The idea is that in the pre-Fall state, Adam could perceive directly the essence of each creature. Today the names we have for animals are only contingent, so "cat" in English and 猫 in Japanese are just labels chosen more or less at random. But the name Adam gave to that creature is intrinsically connected with the essence of the animal -- to its cat-ness. 

Now here I may be using "knowledge" a little bit differently than usual. Normally knowledge is justified true belief. But in the case of Adam and language, knowledge means something conceptual. In this sense, having knowledge doesn't mean that he lacks direct perception of the essence of the thing. It means that he lacks the mental concept -- the ability or the need to interpret, analyze, and consider the thing. If we could directly perceive the thing itself, in this view, conceptual knowledge would be superfluous. 

Likewise, knowledge of good and evil is the same. 

Before the Fall, Adam and Eve were without sin. In Dante's view, sin is not a breaking of rules. It is misdirected desire. This may well be more Greek than Hebrew, but it is well soaked into Christian theology. God is the Good. Without sin -- that is, without misdirected desire -- we naturally want only the Good. We don't have to think about it. 

This is clear at the top of Purgatory, after Dante the character has been cleaned of all sin. At that point, his guide tells him that he should just do whatever he wants. Without the misdirection that sin gives, all spontaneous desire will be properly aimed to the Good. 

So here, too, Adam and Eve don't need knowledge of good and evil because they don't need to conceptualize it, analyze it, or interpret it. They have no need to think about it, because they just naturally do it. When the serpent gets them to sin, they mess up this natural and perfect guide. Suddenly they have the potential to choose badly, to misunderstand, to aim badly, or to lack enthusiasm for what is really good. Instead of instinct they have knowledge. 

You're familiar with the concept of the felix culpa, right? The "happy fault"? A lot of Christians have thought that it is better for us in the long run to have knowledge of good and evil rather than an unthinking direction, even though it causes hardship along the way. They say we are only fully grown up when we go through the struggle of thinking and choosing. So in the long run, Adam and Eve's knowledge was God's plan and better than not having it.