There is an old myth that every cell in the body is replaced every seven years. That would mean that the you that now is is not the you that was seven years ago. In truth, some cells do replace themselves frequently. The lining of the stomach, which is in contact with acids, is replaces about every two days. The skin, which is subject to external forces, may replenish itself every two weeks, although scar tissue may last longer than that, as many of us can attest. Red blood cells last about four months. In contrast bone cells may last ten years. Unfortunately, so do fat cells. And then there are the cells you have with you for life: tooth enamel, the lenses of your eyes, and most of your brain. So while some parts regenerate more rapidly than others, you never replace all the cells in your body. You are still the you of your birth.
Physically, that is true. Spiritually, however, it is possible to say, “I am not who I was.”
That is a good thing. Just as our bodies are subject to decay, our spirits may suffer as well. We were made in the image of God, but spiritually we don’t stay there. We may be born sinless, but we don’t stay that way for very long. “Death passed upon all men, for that all have sinned.” (Rom 5:12) We may not be held accountable for our sins until we are able to acknowledge them and repent, but everyone sins. That is spiritual decay. The ultimate end of that decay, as with the physical, is death.
What fruit had ye then in those things whereof ye are now ashamed? for the end of those things is death. But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life. For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord. (Rom 6:21-23)
We don’t have to die, however. Our physical cells may regenerate, or they may not. Through Christ our spirits may also regenerate.
Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; Which he shed on us abundantly through Jesus Christ our Saviour. (Tit 3:5-6)
That spiritual regeneration is due to a physical death. We attain regeneration by reenacting that death.
We are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection. (Rom 6:4-5)
Peter noticed that there might be some people who do not understand this regeneration. They don’t understand change.
We walked in lasciviousness, lusts, excess of wine, revellings, banquetings, and abominable idolatries: Wherein they think it strange that ye run not with them to the same excess of riot, speaking evil of you: Who shall give account to him that is ready to judge the quick and the dead. (1 Pet 4:3-4)
When others ask why you don’t join them in what you used to do, you can truly answer, “I am not who I was.”