Why is the human race enemies of God?
The human race is enemies of God because Adam's one transgression introduced the old sin nature into the world, and that inherited nature can think nothing but contrary to God's revealed will. Being descendants of Adam, humans are sinners by nature, and this inherent opposition to God makes them enemies.
Les explains that in Romans 5:12 the word 'sin' is singular, referring not to individual acts but to the old sin nature, or old Adam. That fallen nature was inherited from Adam, so every person is a sinner by virtue of being Adam's offspring. This sin nature can think nothing but contrary to the revealed will of God. That inherent opposition is what makes the human race enemies of God—they are contrary to Him. Les illustrates this with a well-trained horse that loses control when someone slips up behind it with cattle prods, showing how people can act when controlled by old Adam.