If we have areas of continuing sin in our lives then there are weaknesses and holes in our breast

‘If we have areas of continuing sin in our lives, then there are weaknesses and holes in our “breastplate of righteousness,” and these are areas in which we are vulnerable to demonic attack.’

Grudem, Wayne A. Systematic Theology: An Introduction to Biblical Doctrine. Zondervan, 2009, loc. 510.

Demons seek to incite us to sin because that sin makes us more vulnerable to their attacks. This is a spiritual equivalent to the principle of Maximizing Your Resources While Minimizing Your Opponent’s Resources and Attack Your Enemy Where They are Most Vulnerable. They seek to increase your vulnerabilities which they then attack. This is one of the reasons sin can seem so cyclical. Our flesh (Ephesians 2:2-3) , which belongs to death and the devil, and still holds the vestiges of our sin (Romans 7:22-23) is seeking to serve the master of this world (John 14:30) by making us vulnerable to the temptations of the devil. This is why we need armor, and it is spiritual armor,…the armor of God.