You're right, not many people seem to understand or believe that Christ is in them.
I would reword that slightly.
Not many people seem to understand how Christ lives in them.
They want to see Christ live in them like a Greyhound bus driver, who says "and leave the driving to me" while they just lay back in their comfortable bus seat doing nothing.
They miss the point that none of Paul's counsel concerning it would have even been necessary if that were so. We have to maintain Christ in us for him to live in us. That means keeping ourselves clean and usable in the temple of Christ's body. We participate in keeping Christ's body holy that the spirit of God be able to dwell among us as body members in the visible body of Christ walking this earth.
Romans 12:1 ¶I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
3 For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith.
4 For as we have many members in one body, and all members have not the same office:
5 So we, being many, are one body in Christ, and every one members one of another.
And this way we cooperate with Christ as he cleanses and beautifies his body that it shines brightly as a witness to this world of the goodness of God's holiness.
1 Corinthians 3:17 "If any man [as an individual] defile the temple of God [which is Christ's body], him shall God destroy; for the temple of God [Christ's body] is holy, which temple ye [ye, plural in the Greek, "all of you together"] are.
You can also let the spirit of the world live in you.
You can also let the spirit of the wicked one live in you.
You can let the spirit of your favorite TV movie star live in you or your favorite football hero live in you. You can put them on and imitate them.
The difference is that if you want God to bless you and your life then you must make only His Son your role model. Then as you put on the image of Christ in your life you will become as a son to God and he will care for you as a son.
Get those other icons and idols out of the way. They only assist you to corrupt the glory of the image of God you were created to be.
Romans 1:22 "Professing themselves to be wise, they became fools,
23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things."
Most are so busy admiring and emulating impressive human heroes that they pattern themselves in their spirit after theirs.
They admire the lion for its courage, the bull for its strength, the hawk for its alertness, the leopard for its stealth, and on and on, while they go about ignoring God. They willingly pattern themselves after the traits of these things rather than patterning themselves after God's Son.
And no wonder, for they could have patterned themselves after God's son if Adam had not sinned. They thus became lost worshiping the creation more than the creator. Those things are impressive but only in a very small way compared to God. Even when they are used honorably they are but a small reflection of God's glory.
Romans 1:25 "Who changed the truth of God into a lie, and worshiped and served the creature more than the Creator, who is blessed for ever. Amen."
If anyone does not understand that this is what this all is all about, then they have not begun to learn yet.
We can be as Romans 12:2 says, "conformed to this world", or we can choose to conform to no one but Christ who is the perfect image of God and the brightness of His [the Father's] glory.
So then as church elect we are faced with how to help others to see this and how to deal with those that resist it (for it is necessary that we protect and maintain the holiness of the body of Christ).
We have every kind of example around us from extreme disregard and permissiveness, to a recreation of harsh Pharisaical type rule.
Yet, the scripture is complete in telling us how to do it properly.