Mankind's Problem: Sin
Man chose to sin against God rather than serve him. The Bible Says...

"For all have sinned and fall short of the glory of God"
Romans 3:23

What does it mean to sin against God? Sin is breaking God's law:

"Everyone who practices sin also practices lawlessness; and sin is lawlessness."
1 John 3:4

We do this by doing what God has commanded us not to do, or by not doing what God has told us to do.