What does the Bible say about internal struggles within believers?

In Galatians 5:17, Paul explains that the flesh lusts against the Spirit, and the Spirit against the flesh, creating a constant struggle within a believer. This internal battle is reflective of the two natures present in every Christian: the old sinful nature resistant to God's ways and the new divine nature that desires to follow Him. This warfare highlights the reality of salvation; the spiritual conflict indicates that a person is indeed alive in Christ, as they experience the opposing desires of sin and holiness.