How do we know that believers cannot sin?

The doctrine that believers cannot sin pertains to the holy nature they receive through being born again. 1 John 3:9 asserts emphatically that 'Whosoever is born of God doth not commit sin,' which suggests that the divine seed, representing God's nature, abides within them. This holy nature is not subject to sin, contrasting sharply with their sinful flesh that still exists. Therefore, while believers may fall into sin through their old, sinful nature, their new spiritual existence is fundamentally incapable of sinning, which assures them of their standing before God and the efficacy of Christ’s redemptive work.