Demon possession possible?
In the most simplest terms yes. By that I mean ~
We are natural born creatures ie our first birth that is applicable to every human that has been born alive, on this earth.
We have two choices;
Become born again - which in it's simplest term is to receive the Spirit of God within us - which in it's simplest term is to GIVE, allow, authorize God to have complete possession of our WHOLE being (body, soul and spirit).
The second choice is to DENY, refuse, NOT GIVE, allow nor authorize God to have complete possession of our WHOLE being.
The second choice has a "caveat". Meaning, it is a "by default choice". Meaning, "IF" a human does not "Give, allow, authorize God" to have complete possession of our WHOLE being, BY DEFAULT, we are electing the second choice, which is Denying, refusing, Not giving, Not allowing, Not authorizing God to have complete possession of our WHOLE being.
Thus the question remains, IF it is not God who is given possession of our whole being, THEN WHAT is "by default" now in opportunity to the possession of our being?
And aside from the "default" caveat ~ IS there an option FOR a human to outright Give, allow, authorize, SOMETHING other than God to have possession of their being? Yes.
In either case it is the intermingling of a human WITH a spirit and spirit WITH a human.
So HOW would another human KNOW, which spirit intermingles with an other human?
Scripture tells us -
We are all naturally born in sin ~ Sin is that which Stands Against God, resulting in the separation of us from God.
God has provided us a WAY "to" become forgiven and cleansed of our separation from God and reconciled unto God.
"IF" we choose such path/way/option; it is Gods Spirit that shall possess us.
"IF" we choose to submit to Satan's spirit; it is Satan's spirit that shall posses us.
"IF" during our natural life-time, we make no choice, by default, we give up our option to CHOOSE Gods Spirit to possess us.
So, we know who is "possessed" by which spirit/Spirit, by our own confession of the truth in our hearts, that God "knows" and for humans to know, it is expelled, ie spoken, out of our mouth.
And then there is the knowledge, that men are not privy to peek into a mans truth in an other's heart, and that men are LIARS. Thus, one man attempting to figure out which spirit another man IS subject to;
requires observation of the others mans "fruits"....ie speech and actions. And even then, at any point in a mans natural life time, such man has the option to submit unto Gods Spirit.
Meaning, what you may know of another man one day, may not be the same of what applies to him the next day.
The bottom line is ~ God is thee Eternal Spirit. Satan is a created spirit. Thus they are not opposites, however, either spirit can possess a man and man has an option of WHICH spirit shall possess him.