This is why the Bible describes a saved person as a new creature. Spiritually, a Christian can no longer sin because his spiritual nature has been changed to righteousness. This spiritual transformation is called the new birth. It is a new beginning with a new nature as a child of God. Having a righteous nature prevents sin, and if you cannot sin, you cannot die.
Christians have two natures.
The Bible refers to the physical body as the outward man, and the soul and spirit as the inward man. The nature of the body remains unchanged and is, therefore, unrighteous and sinful. However, the inward man is spiritual and its nature is changed to righteousness. Because the body is inclined to sin and the inward man desires to do right, Christians are continuously subject to the struggle between the two opposing natures. Nonetheless, because the inward, spiritual man is righteous, it will never sin regardless of what the body does.