Few of us, even as Christians, realize the continual danger that this ultimate temptation constitutes. It is the delicious area of being 'as God' ... judging and deciding, usually about others, those delicate and intangible things that can only be known by Him who alone is Judge. Those who eat from this tree (which is death) presume to be aiding God and rendering a service that only they in their 'superior' knowledge and concern can perform! This is ultimate deception as it is ultimate temptation, and if we could see it aright, ultimate evil. I quote from Karl Barth's discussion in IV.1 of his Church Dogmatics, pp.450-453. All italics and emphases are my own.
[Vain man does not know that] the divine knowledge of good and evil has already preceded him; this involves a foolish over-estimation of himself, as though he is the one who can stand over that alternative and exercise the function of an Atlas bearing and holding together the great building of the universe ... He forgets that there is only One who can do that, and that he is not that One ... He cannot abase himself worse than by desiring this knowledge and wanting to make himself judge.
But it also means that man wills that which is objectively evil. Of course he does not believe that he is doing that which is evil but that which is good, that which is commanded and necessary, and therefore the best of all. In this form of sin, there can be no question of any indolence or frivolity ... On the contrary, there is a pathetic earnestness, an outward air of the most serious responsibility, the most stringent sense of duty, the most militant virtue. As judge of good and evil, man wants to stand at God's side in defense of the cosmos ... against the invasion of chaos and disorder and wrong ... himself a cherub with drawn sword at the gate of Paradise, or at the very least a watchman on the walls of Jerusalem! He wants to spring into the breach, safeguarding the right with his own affirmation and negation, with his own building up and tearing down, successfully maintaining the cause of God and the cause of man! And it is a really shattering fact that he is mistaken in all of this, that he ought not to do it at any cost, because he lets hell loose by doing it ... It is an unleashing of evil when the man to whom it does not belong to distinguish evil from good and good from evil, who is not asked to do so, who is prevented and forbidden, still wants to be the man who can and pretends that he is this strong man!
I am already choosing wrong when I think that I know and ought to decide what is right, and I am doing wrong when I try to accomplish that which I have chosen as right. I am already putting myself in the wrong with others, and doing them wrong when ... I confront them as the one who is right, wanting to break over them as the great crisis. For when I do this I divide myself, and I break fellowship between myself and others. To use the words of the serpent in Genesis 3, when our eyes are opened to the possibility of our own exaltation in judgment, we become truly blind to what is right or wrong...sowing and reaping discord as the children of discord ... but our ignorance of God means that we conceive and hold and constantly return to the stupid and ridiculous notion that we can take our place at His side - as though He were a God who needed to be completed by us, and helped by our counsel and assistance. We are deceived in this, and in this self-deception, in the delusion that we are doing good and avoiding evil, we actually eschew the good and do evil.