Animals, birds, fish and insects can’t help being what they are. Their behaviour is governed by instinct. You can train some animals and birds to some degree, but ultimately their DNA dictates their behaviour. So these lesser forms of life cannot sin.
However, mankind is unique. Physically, he’s an animal, but spiritually he’s created in God’s image. God has given him free choice. Man can choose between good and evil. He has a rational mind and he has a will. Therefore, man has the potential to change.
Nevertheless, it can be extremely difficult to change. There are things about me that may never change, yet I have changed in profound ways. I have battled against certain sinful inclinations for years and made absolutely no progress. Occasionally, I even believed that I was a hopeless case. Then I changed. It was remarkable and I amazed myself. I know that it was God who ultimately made the change. However, I believe He would not have acted had I not DEEPLY DESIRED Him to do so.
We can serve God and obey His commandments IF WE REALLY WANT TO. Ultimately, people do not give up sin because 1) they do not want to change badly enough and 2) they do not ask God PERSISTENTLY to change them. What a man desires most deeply in his heart will determine his destiny. If you love evil, you will never give up sin. Even if you sometimes repent of sinful actions, you will continue to do them because your sinful desire compels you to sin. If, on the other hand, your deepest desire is to obey God, even if you repeatedly commit the same sin, you will change over time. When a profound change finally comes over you, you will realize that it was God who did it. God demands that you do your one percent, but He will do the other 99 percent. This supernatural action by God, which ultimately saves us from damnation, is what is meant by grace.
This leads us to the question, “What determines the deepest desire of a man?” I believe that each man has good and evil desires. He must choose which he will allow to dominate his heart. The choice is yours and mine to make, every day.