Regarding demons within a Christian. The question is, are they truly spirit filled to have demons possessing? Only God possesses his own, he does not share, we choose who we shall serve, but the choice must be followed by God's filling us with his Spirit. No Spirit, and we are in control choosing one or the other, without God's confirmation. The saving of a soul is God's work within. If the demons are within, we may say one thing and the evidence says another.
John 1:12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:
John 1:13 Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
1 John 4:3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
1 John 4:4 Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.
The devil can put thoughts in our mind. So, we must cast down these thoughts that are not of our Lord. We know the word, we do not add to it, or cause questions of strife. Yes, the Lord has the answers, we call on him to show us in his word and to bring peace. Too many get too far from the whole word of God, taking one scripture and magnifying it to be more than intended. Positive confession I see this way. We can confess Gods promises and hold onto them, but many speak as if they are God saying things to make them as if God said them. Soon they do not know God's thoughts, but their own thoughts making great sounding words, with no power. We speak God's word, the power is in the word. The Spirit brings the word to life, to our understanding. We are not just to entertain, nor should we bore with our own words. The word is God speaking to us, I never tire of hearing God's word spoken through His Spirit filled person.