God knows their secret motives and thoughts. They think their own natural goodness and decency are enough to merit the right to enter heaven. The God who sent Jesus is the God to whom we owe everything. He even gives us the grace to obey. Therefore, our obedience earns us no merit with Him.
Obedience is just common sense - like getting in out of a storm.
And it is our only way of saying thanks to a God we love and trust. With such obedience, we flourish. Because God is the power of life. You can't resist the power and energy of life without getting hurt. We can see this illustrated in the laws of nature.