Where does it say that Christians can't be demonised
The Spirit is the guarantee [the first installment, the pledge, a foretaste] of our inheritance until the redemption of God’s own [purchased] possession [His believers], to the praise of His glory.
Ephesians 1:14 AMP
But you are a chosen race, a royal priesthood, a consecrated nation, a [special] people for God’s own possession, so that you may proclaim the excellencies [the wonderful deeds and virtues and perfections] of Him who called you out of darkness into His marvelous light.
1 Peter 2:9 AMP
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Little children (believers, dear ones), you are of God and you belong to Him and have [already] overcome them [the agents of the antichrist]; because He who is in you is greater than he (Satan) who is in the world [of sinful mankind].
1 John 4:4 AMP
We were delivered at point of our conversion
For He has rescued us and has drawn us to Himself from the dominion of darkness, and has transferred us to the kingdom of His beloved Son, in whom we have redemption [because of His sacrifice, resulting in] the forgiveness of our sins [and the cancellation of sins’ penalty].
Colossians 1:13-14 AMP