From what I know of the story of Dr. Jeckyll and Mr. Hyde, it almost describes the condition of every Christian, except that the Christian life begins with the Hyde part, the old nature that is full of sin and doomed to perish.
The other difference is that we do not have a Dr. Jeckyll part. Instead, we need a new nature from God. For those who follow Jesus Christ, that new nature and the old are housed in one body. While our behavior seems not as extreme as that famous story, as with that story, only one nature at a time can be in control.
This duplicity begins with a new birth. The first New Testament character to hear about it was a man named Nicodemus. Of course he had questions, the same ones that people still utter . . .
“Nicodemus said to him, ‘How can a man be born when he is old? Can he enter a second time into his mother’s womb and be born?’ Jesus answered, ‘Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, “You must be born again.”’” (John 3:4–7)
Jesus did not explain how it happens except to say that this new birth is a work of the Holy Spirit and is like the wind; He moves where He wishes and we may be aware of His movements, but no one can see Him.
The rest of the answer is given in later passages where Paul describes the results of having a new, reborn nature. In those who have it, there is a death to the old ways, a death to trusting in anything else but Jesus Christ for salvation or for doing the will of God. As Chambers says, along with this dying comes “conscious repentance and unconscious holiness.”
New birth is about receiving Jesus (John 1:12). It is not a mimicking of the Christian life, but a genuine connection with the Lord, an new and personal knowledge of Him and an awareness of a spiritual realm previously unseen and not experienced.
It is also about receiving the very nature of God. This changes a person’s perspective about the things of God. Before, all of His precepts seemed a restriction, an unwanted burden, but when He gives His own life to us, we see His realm with His eyes. We see His goodness and His love written in everything He says.
This changes behavior. Because of this new birth my attitude toward sin changed and also the power to stop doing it. Although my sinful nature is there lurking in the background, ready to jump in should I rebel against God or believe one of Satan’s lies (both seem to go hand in hand), this new life is strong and able to say no that those things that previously held me prisoner.
1 John 3:9 says, “No one born of God makes a practice of sinning, for God’s seed abides in him, and he cannot keep on sinning because he has been born of God.”
The answer to the question of how can a person be reborn even when old is that God is able to accomplish this in the human heart. Salvation from sin is His work. New life comes from Him. No one can step out of that old nature to new life apart from the saving power of Jesus Christ.
That said, no Christian can overcome sin without that same saving power. I can struggle against it on my own, but without the glorious power of Jesus given in new life, my efforts to save myself are the same as they were before that new birth happened. Without Him, I could not do it then and I cannot do it now.
The only way to live as God commands is to reject that old life and yield to the power of the new:
“But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth. Do not lie to one another, seeing that you have put off the old self with its practices and have put on the new self, which is being renewed in knowledge after the image of its creator . . . letting the peace of Christ rule in your hearts . . . .” (Colossians 3:8–15)
New birth equals new life. God makes it happen and enables me to live it out!