Jesus said "by their fruits ye shall know them." We take this to mean that a tree is bad because its fruit is bad, but in fact it's not a statement about what determines the objective condition of the tree. It's a statement about the empirical basis for our knowledge of its condition. If the tree bears bad fruit, it's a bad tree. Any fruit that comes from it is bad, since a bad tree cannot (does not have the power to) bear good fruit.
Christ's assertion seems contrary to possible human experience. Don't we find bad cherries growing side by side with good ones? If we remember the story of Adam's fall from Grace, however, we can easily understand Christ's instruction. Eve looked at the fruit of the tree of knowledge and judged it to be good based on her (human) faculties of perception and understanding (it looked ripe and luscious and good for food.) But according to the word of God, the tree was bad. Eating its fruit meant death. God's word reflected the objective nature of the tree with respect to Adam and Eve. For them, it was a tree of death. The badness of the tree was not caused by the fruit. Rather, the objective nature of the tree gave rise to the badness of the fruit.
So how can we know that a tree is bad; by judging its fruit in light of God's word rather than relying on information derived exclusively from our own faculties. Using Obama as an example, one of his fruits appears to be making sure that abortion and infanticide are sanctioned as right and lawful. God's will for us condemns the taking of innocent human life. Obama's fruit is contrary to God's will, and therefore bad. But a good tree cannot bear bad fruit. Therefore Obama is a bad tree. This means that all his fruit is objectively bad; however it may look to us. We cannot cherry pick the fruit of a bad tree once we have determined its objective condition. This puts the lie to the actions of those so-called Christian leaders who think we can support Obama when he does what is good, and oppose him when he does wrong. According to Christ's criterion, no fruit that he produces can be good. Though it looks good, and may produce what seem to us to be good results, according to Christ's rule of moral reasoning, his fruit is bad because he is bad.
If we had the time to step back and take in the whole picture, we would better understand the sense of this. Back in the 1930s Adolf Hitler helped revive the German economy, reduce unemployment, improve the infrastructure, encourage scientific research and development and many other "good" things. Each such "good" policy increased his popularity and expanded his support, contributing to the cult of adulation that empowered him to implement policies of holocaust and aggression. Those policies morally depraved the German nation, and plunged the world into total war. Objectively speaking, the elements of German society that accepted what they saw as good results from the Fuhrer made Eve's mistake. Of course, they didn't realize the truth until Germany lay in ruins, defeated and literally demoralized.
I think the difference between God's knowledge and ours lies in this: he has intrinsic knowledge of the objective condition of each tree at every moment, even before its fruit appears. Our objective knowledge depends on our experience. In his word he tries to share the fruits of his intrinsic knowledge with us. But since his guidance arises from a way of knowing that transcends the preconditions and categories of our understanding, we cannot know with certainty that his guidance is correct until our experience verifies it. Of course, if the experience involves our destruction, the satisfaction of certainty won't last for long. Is this one reason the Psalmist declares:
It is better to trust in the LORD than to put confidence in man.
It is better to trust in the LORD than to put confidence in princes