Every human inherits a sin nature from Adam, which corrupts thoughts and behaviors in opposition to God (Jer 17:9; Rom 5:12). This nature manifests in both self-righteous legalism and lawless indulgence, leaving unbelievers spiritually dead and incapable of pleasing God (Rom 8:7-8; Eph 2:1-3). At salvation, believers receive a new nature, created in righteousness, yet the old sin nature remains, leading to an ongoing battle between the flesh and the Spirit (Rom 7:14-25; Gal 5:16-17).