What does the Bible say about the two natures of a believer?

In 1 Corinthians 15, Paul explains that the first Adam was made a living soul, but the last Adam, Christ, was made a quickening spirit. This indicates that the believer has two natures: the old man, which is bound to sin, and the new man, which is perfect in Christ. Romans 7 further illustrates this struggle, as Paul laments the conflict between his desires to do good and the sin that dwells in him. Believers are aware of their dual existence, constantly wrestling between these natures while finding hope in their identity in Christ.