As Christians we are definitely in spiritual battles with demons; that doesn't mean they can
possess us.
There's spiritual
oppression but demons can't
indwell a believer. The Holy Spirit resides in us, He has control. Demonic possession would mean demons have control over us. By oppressing a Christian they can only (albeit heavily) tempt and influence us.
Notice in Eph 6 there is no instruction - or mention - of casting demons out.
James 4:7
Submit yourselves therefore to God.
Resist the devil, and
he will flee from you.
Every believer should know how to fight the enemy, but since we are indwelt by the Holy Spirit Himself, there's no
possession.
I claim this verse a lot -
1 John 4:4 Little children,
you are from God and
have overcome them, for
he who is in you is greater than he who is in the world.
"He who is in you" = the Holy Spirit - is greater than "he who is in the world" = satan.
Does that make sense?