A major characteristic of the human spirit is that it is habitually self-centered rather than God-centered. A simple example illustrates how it became this way. In Genesis 1:31 God takes satisfaction in all He had made, declaring it "very good." Included in this is Adam and Eve's nature, as they were already created by this time.
Thus, at the beginning, mankind's nature was not corrupted by contact with this world. Genesis 3 records the episode of their confrontation with Satan that began the evil transformation of their basic nature. God did not create their nature as evil, but it became evil through the influence of another spirit that they chose to follow without any intervention from their Creator.
The same process continues to this day, as each of us is born into this world and comes under the influence of the same spirit that influenced Adam and Eve to turn from God. We are all born with a slight pull toward self, but not with the evil that eventually develops and manifests itself in our conduct. Evil is not - cannot be - passed on through procreation, but it is fashioned anew by the spirit of the age into which each person is born. It is a converted parent's responsibility to God and to his children to ensure the right spirit dominates his home so the children can be properly nurtured.
People in the world understand this to some extent when they observe with maxims like, "The apple doesn't fall very far from the tree," "Like father, like son," or "Like mother, like daughter." This world's Christians, to avoid responsibility for their evil, have blamed God for creating us this way. But God did not make us this way. Mankind, represented by Adam and Eve, chose to become this way, and all of their descendants, including us, have chosen the same path under the influence of the same evil spirit who offered Adam and Eve the choice. This accounts for the course of this world.
Jeremiah 17:9 shows us how evil God judges the source of our unconverted motivations to be: "The heart is deceitful above all things, and desperately wicked; who can know it?" The Revised Standard Version translates this as, "The natural heart of man is desperately corrupt; incurably sick." It is so bad, so evil, it cannot be salvaged by repairing it! It must be completely replaced. This is what the conversion process - our calling, repentance, justification, and sanctification - accomplishes.
We need to understand more completely why this aspect of God's command to flee Babylon is so important. We can be easily deceived about it, misunderstanding why God says the human heart is incurably sick. In Luke 11:13, Jesus makes an easily overlooked comment: "If you then, being evil, know how to give good gifts to your children, how much more will your heavenly Father give the Holy Spirit to those who ask Him!" The way He says this implies that those before Him were thoroughly, not partly, evil.
He flat out calls them evil! There is no equivocation, no modification of this verse in the heart of the sermon on the mount. Jesus Himself was called "good" in Matthew 19:16, but He immediately corrects the speaker, saying, "No one is good but One, that is, God." This is God's assessment of human nature, not man's.
Jesus is saying that, just because human nature knows how to and actually does some good things, it does not alter the fact that it is still incurably evil. Our pride tends to blunt God's assessment, rising to defend us from the condemnation of what we are compared to, the standard - God.