For I know that no good thing dwells in me, that is, in my flesh. Romans 7:18.
Can a genuine Christian be demonized?
I would argue, yes:
1. When we are born again, we come into our relationship with Christ with various injuries, misunderstandings, and demonic strongholds of lying spirits. We then begin the ongoing process of sanctification (1 Thes. 5:23), whereby we are freed from these. This deep, gradual process of deliverance and exchange includes the renewing of our minds to our true identity in Christ —as we cooperate with the Holy Spirit to discern and receive scriptural promises.
2. Yet even as Christians, we can still willfully sin, and thus we can again open the door to the satanic to come into our lives—
Do not give the devil a foothold. Ephesians 4:27.
The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons. 1 Timothy 4:1.
Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour. 1 Peter 5:8.
However, I do not believe that a genuine Christian, even through willful sin (or through demonization), can lose his salvation—
And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand. John 10:28-29.
And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Ephesians 4:30.
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. Galatians 2:20.
For you died, and your life is hidden with Christ in God. Colossians 3:3.
Building on the truths of Galatians 2:20 and Colossians 3:3—in order to obtain deliverance from the world, the flesh, and the devil—please consider this seven minute excerpt from Pastor Joseph Prince of New Creation Church in Singapore:
PS To combat lying spirits, in addition to renewing our minds through grace teachings about our true identity in Christ, I would also recommend seeking the baptism in the Holy Spirit, and praying in the Spirit (tongues).