This truth can be explained in this way: you can take a vessel of any metal, and reform it, change its shape and design. You can melt and mold it into quite a variety of forms. But you cannot change its essential nature. If it is of iron, or brass, or silver, it will remain the same metal.
Not until you can change the iron into brass, the brass into silver, the silver into gold, can you change by your own effort your sinful nature and be a partaker of the divine nature.
The outward life may be reformed, but the soul remains the same in essential quality, in the sight of God.
Have you been born again?