Romans 6:6
We know that our old (unrenewed) self was nailed to the cross with Him in order that [our] body [which is the instrument] of sin might be made ineffective and inactive for evil, that we might no longer be the slaves of sin.
Our old man is everything that we were before we became Christians.
Ephesians 4:22 throw off your old sinful nature and your former way of life, which is corrupted by lust and deception. 23 Instead, let the Spirit renew your thoughts and attitudes. 24 Put on your new nature, created to be like God—truly righteous and holy.
Colossians 3:9Don’t lie to each other, for you have stripped off your old sinful nature and all its wicked deeds. 10 Put on your new nature, and be renewed as you learn to know your Creator and become like him.
The new man is not perfect. We still sin because we have indwelling sin in our mortal bodies
Gen 2 (NKJV)
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¹⁶ And the Lord God commanded the man, saying, “Of every tree of the garden you may freely eat;
¹⁷ but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.”
Gen 3 (NKJV)
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¹ Now the serpent was more cunning than any beast of the field which the Lord God had made. And he said to the woman, “Has God indeed said, ‘You shall not eat of every tree of the garden’?”
² And the woman said to the serpent, “We may eat the fruit of the trees of the garden;
³ but of the fruit of the tree which is in the midst of the garden, God has said, ‘You shall not eat it, nor shall you touch it, lest you die.’ “
⁴ Then the serpent said to the woman, “You will not surely die.
⁵ For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.”
⁶ So when the woman saw that the tree was good for food, that it was pleasant to the eyes, and a tree desirable to make one wise, she took of its fruit and ate. She also gave to her husband with her, and he ate.
⁷ Then the eyes of both of them were opened, and they knew that they were naked; and they sewed fig leaves together and made themselves coverings.
⁸ And they heard the sound of the Lord God walking in the garden in the cool of the day, and Adam and his wife hid themselves from the presence of the Lord God among the trees of the garden.
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²² Then the Lord God said, “Behold, the man has become like one of Us, to know good and evil. And now, lest he put out his hand and take also of the tree of life, and eat, and live forever”
Romans 7:13 But how can that be? Did the law, which is good, cause my death? Of course not! Sin used what was good to bring about my condemnation to death. So we can see how terrible sin really is. It uses God’s good commands for its own evil purposes.
14 So the trouble is not with the law, for it is spiritual and good. The trouble is with me, for I am all too human, a slave to sin. 15 I don’t really understand myself, for I want to do what is right, but I don’t do it. Instead, I do what I hate. 16 But if I know that what I am doing is wrong, this shows that I agree that the law is good.
17 So I am not the one doing wrong; it is sin living in me that does it.
18 And I know that nothing good lives in me, that is, in my sinful nature. I want to do what is right, but I can’t.
19 I want to do what is good, but I don’t. I don’t want to do what is wrong, but I do it anyway.
20 But if I do what I don’t want to do, I am not really the one doing wrong; it is sin living in me that does it.
21 I have discovered this principle of life—that when I want to do what is right, I inevitably do what is wrong.
22 I love God’s law with all my heart.
23 But there is another power within me that is at war with my mind. This power makes me a slave to the sin that is still within me.
24 Oh, what a miserable person I am! Who will free me from this life that is dominated by sin and death?
25 Thank God! The answer is in Jesus Christ our Lord. So you see how it is: In my mind I really want to obey God’s law, but because of my sinful nature I am a slave to sin.
But we are in the process of renewal Ephesians 4; Colossians 3. Thus we have the answer to the question about whether a Christian can still live in sin. We cannot live as we once did because the “old man” was crucified with Christ. In Christ the believer is a “new creation”
2 corithians5:17 This means that anyone who belongs to Christ has become a new person. The old life is gone; a new life has begun!