How can Christians have demons if they have the Holy Spirit?  They can't coexist in the same space.

When we ask for Jesus to come into our hearts, a spiritual connection is made between the Holy Spirit and our spirit.  No room for a demon.

Our soul, however, is a battleground for the rest of our lives.  Pre-Christian, we have an old nature or personality in our soul that has collected demons and sin.  When we become Christians, we have a small new nature or personality that is Christian formed in our soul.  It has a connection to the Holy Spirit.  Both the old nature and the new nature are at odds to each other.  They have different world views, attitudes and priorities.

The turf of the soul can be gained by the new nature by:
- reading the Bible
- praying to God
- reading or watching things that reflect Christian attitudes, identity or world views.
- confessing the sins of our whole soul, renouncing the attitudes and actions, commanding any demons that might exist to leave and dedicating that area in our life to Jesus. (sanctification)
- asking Jesus to speak Truth into our traumas of the past.

The turf of the soul can be lost to the new nature by:
- submitting to demonic advice or authority
- opening our soul to whatever in mind-blanking meditation
- refusing to deal with traumas but stuffing them into our subconscious that is under control by the old nature
- reading or watching fiction or non-fiction that promotes demonic attitudes, identity or world views.
- submitting to the laying on of hands by someone demonized.