Moshe Halbertal, People of the Book: Canon, Meaning, and Authority (Cambridge: Harvard University Press, 1997) 43.
Halbertal discusses charitable and not-so-charitable ways that interpreters have read the Hebrew Bible, and he quotes a Gnostic interpretation of Genesis 3:22 as an example of an uncharitable approach. After Adam and Eve eat the forbidden fruit and hear from God about their punishment, God says, "See, the man has become like one of us, knowing good and evil; and now, he might reach out his hand and take also from the tree of life, and eat, and live forever" (NRSV). The Genesis story then says that God expelled Adam and Eve from the Garden of Eden. The Gnostic "On the Origins of the World" (second-third centuries C.E.) renders God's words as follows:
"Behold, Adam has come to be like one of us, so that he knows the difference between the light and the darkness. Now perhaps he will be deceived as in the case of the tree of knowledge and also will come to the tree of life and eat from it and become immortal and become lord and despise us and disdain us in all our glory. Then he will denounce us along with our universe, come let us expel him from Paradise."
According to the Gnostics, the God of the Old Testament who created the cosmos was a sinister sub-deity seeking to enslave the benevolent top God. This sub-deity conspired with his angels to expel Adam from the Garden because he thought Adam would despise him: Adam now knew the difference between light and darkness, like the Demiurge; he could become immortal, like the Demiurge. Eventually, he'd vaunt himself against the Demiurge, who wouldn't be as superior to Adam as he once was. According to this Gnostic interpretation, the Demiurge expelled Adam from Eden out of insecurity.
This was an uncharitable interpretation, one that placed the God of the Hebrew Bible in a bad light. I checked out some charitable ones, and I was interested in the results. The three I looked at were Philo, Genesis Rabbah, and a few ancient Christian interpretations.
I didn't entirely understand Philo's interpretation of Genesis 3:22. For "Adam has become as one of us," Philo just says that God perceives virtue differently than we do, presumably because his vision is clearer. On the expulsion of Adam from Eden, Philo emphatically denies that God did this out of envy, for he says God deprived Adam and Eve of eternal life because wicked people would be miserable if they lived forever (Questions and Answers on Genesis I:54-55).
The rabbinic Genesis Rabbah 21 offers a few interpretations of "Man has become like one of us." The passage can also be translated "Behold, the man was like one of us." In this case, God is lamenting man's fall: one minute, man was like God, made in God's image, knowing good and evil, with the potential to live forever. The next minute, after his sin, man is corrupt and degraded, so ashamed of himself that he covers himself up with fig leaves and hides from God. This rabbinic interpretation essentially says "How the mighty have fallen."
In another interpretation, a rabbi changes the vowels and makes the word "was" or "became" an imperative: "Let the man become like one of us, to know good and evil." According to this view, God will one day lift the curse of Adam, meaning people will no longer experience death. At this time, God will allow human being to become like God, knowing good and evil and living forever.
I didn't read every single ancient Christian interpretation, but I found some good ones here. The common view I saw was that God was being ironic when he said Adam had become like him. The serpent promised Eve that she and Adam would become like God once they ate the fruit, and, according to Christian interpreters, this did not happen, for Adam and Eve were quite pathetic after they did so (e.g., clothing themselves with fig leaves, hiding from God). So, in this view, God is sarcastically saying, "The man has become like us, all right. Look at him!" God doesn't really mean it but is being sarcastic.
The historical-critical interpretations that I've heard usually sound like the Gnostic view: God had certain divine prerogatives, and man usurped them when he ate the forbidden fruit. I can do more reading on this topic, I admit, especially of the ancient Near Eastern parallels. But I think that ancient interpreters do well to point out how pathetic Adam and Eve sank after their sin. Were they seriously like God right after they ate the fruit, with all their fear and shame?