Illustration. Many wonder why we still sin if we are new in Christ. Well there was a news story long ago where there was a young woman who was horribly neglected by her parents. When police found her, she was living in a dog house with dogs. She crawled on all fours like a dog, tried to bark like a dog. Yet she was so used to being treated like a dog, in her mimd she thought she was a dog.
Now that is how she was living all her life. It probably took extensive time and professional intervention to help her become like a normal human being.
Yet spiritually, we too have gotten so used to sinning by nature, we do not just unlearn that over night. Christ saves us the moment we believe the gospel, and continues to work with us to restore us from the effects sin has had on us all our lives up until we get saved. So through persevering we unlearn old patterns and behaviors and grow to more and more reflect our true new identity