Can demons infiltrate Christians? The late Derek Prince suggested that they can, and in the book, Rules of Engagement: Preparing for Your Role in the Spiritual Battle, he explains how:
In James 3:15 the apostle speaks about a form of wisdom which does not descend from above, but is earthly, sensual [soulish], demonic. James depicts a downward slide in three successive stages: from the earthly to the soulish to the demonic.
When Christians become earthly they lose the vision of eternity. They cannot see beyond the things of this life: success, pleasure, wealth, physical health. They are only interested in what their faith will do for them in this life! Concerning such people Paul says: If in this life only we have hope in Christ, we are of all men the most pitiable (1 Corinthians 15:19). Christians like that often consider themselves prosperous and successful. God considers them pitiable.
After the earthly, the next stage is soulish. To be soulish is to be egocentric, self-centered. For such people, the Christian faith is a way to get what they want out of life. They suppose that godliness is a means of gain (1 Timothy 6:5).
The soulish opens the way for the demonic. This is one main way in which demons infiltrate the church. The question is often asked: Do Christians ever need deliverance from demons? The words of James provide a clear answer. This downward slide from the earthly to the soulish to the demonic exposes both individual believers and whole congregations to the activities of demons.