I’ve gotten quite a few questions about this, so I figured I’d discuss this here, since a lot of Christians are held back because they don’t understand the difference between the flesh and the old man. Many people use these terms interchangeably, and they are not the same thing!
If you want proof of that statement, consider this: Jesus had flesh that was weak and did not want to do the will of God by being tortured to death for our sins. He cried to God that if there was any other way, He wanted a way out. But when God didn’t give Him another way, His flesh had to submit to His spirit, which was willing even though His flesh was weak. So here’s the problem with saying that the flesh is “the old man” or “the sinful nature” – if you believe they’re the same, then you believe that Jesus had an “old man” and a “sinful nature”, which He assuredly did not!
Adding to the confusion for readers of the NIV version is that it constantly paraphrases the word “flesh” in the original as “the sinful nature”. (The NIV is partly a paraphrase, not a strict translation.) So it would leave you with the impression that you are fighting a sinful nature on the inside of yourself. But the main point of redemption was making man able to be a child of God, a new creation, not just a forgiven sinner. The old man has passed away and all things have become new. If you’re born again, you have a new spirit that does NOT have a sinful nature, which replaced the old spirit that DID have a sinful nature. You are not two spirits, and old one and a new one, sharing the same body! You are a new creation in Christ. You are a reborn spirit. However, you still have the same body you did when you were a sinner, and your natural body does not know how to make spiritual choices. Your body is not born again. You need your body to contact the natural realm – without it you’re gone! But like Paul, you have to keep your body in submission to the real you, who is a spirit man, a righteous, holy “new man” created in God’s image.