The Apostle Paul is addressing the Galatians concerning his relationship with Christ and his daily walk as a Christian. He makes it plain that none of it is possible without the faith of Jesus Christ and the abiding presence and power of the Holy Spirit.
' I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.'
This statement should also be the affirmation of every Christian; and for those who are struggling to live by the faith of the Son of God, they should seek the face of God that he may grant them repentance for walking in the flesh and not in the Spirit.
In most instances there is a measure of unbelief or misunderstanding in some of the doctrines of the Apostles, as well as lying, anger, strife and things of that nature. All these can cause the believer to waver thus opening doors for the devil.
Ephesians 4:27 says, Neither give place to the devil.
Have a look.