What does the Bible say about the warfare within the believer?

Romans 7 outlines the struggle of the believer, depicting it as a warfare between two opposing natures: the old man, which is corrupt and sinful, and the new man, which is created in righteousness. This 'warfare' signifies that there is no peace or truce in the conflict as both natures vie for dominance. The Apostle Paul captures this tension, acknowledging that even when he desires to do good, he finds evil present in him, indicating that believers will experience this ongoing internal struggle as long as they live in this world.