Brothers and sisters: Those who are in the flesh cannot please God. But you are not in the flesh; on the contrary, you are in the spirit, if only the Spirit of God dwells in you. Whoever does not have the Spirit of Christ does not belong to him. But if Christ is in you, although the body is dead because of sin, the spirit is alive because of righteousness. If the Spirit of the one who raised Jesus from the dead dwells in you, the one who raised Christ from the dead will give life to your mortal bodies also, through his Spirit dwelling in you.Before we get into what "in the flesh"means, and the difference between 'spirit' and 'the Spirit of Christ', note how different the emphasis here is from most Western Christians' preoccupations as Christians. Most tend to think of the attainment of eternal life in juridical or even forensic terms: you believe P, Q, etc. and do A, B, etc. so as to get to heaven; refuse, and you don't get to heaven. That's an immature moral universe to live in, and it's very thin gruel. Unfortified, it causes sterility, boredom, and resentment that in turn make "backsliding" inevitable. What Paul is really on about is a new, abundant life: one infinitely more valuable, joyful, and lasting than life as we normally experience it, which is life "in the flesh." The doctrines and rules, and the deeds that go along with them, are merely expressions of that new life.
To be "in the flesh" in the sense contrary to "in the spirit" does not mean to be disembodied or to live as though the body doesn't matter. The body matters a great deal, else God would not have become truly incarnate and lived as such. The point of the things of this world, including the body and all that goes with it, is to be vehicles for what God is trying to make of us: little Christs, gods in fact, whose origin and destiny far transcends this world. To be "in the flesh" is to fail to get that point and live as though the things of this world had value and meaning apart from it. That way leads to death—eternal death, in fact. To be "in the spirit" is to get the point and live accordingly "in the world but not of it."
The Holy Spirit, who is God, empowers our souls, which are spiritual, to do just that. To the extent we accept and use that power, our very bodies and selves are transformed along with our own respective corners of the universe. Isn't that so much more exciting, and motivating, than wealth, sex, and power, which are so ephemeral? In fact, isn't it so much more realistic than the serpent's deceptive promise in the Garden of Eden: "You shall be as gods, knowing good and evil"?
In the form of the serpent, Satan promised that to our first parents as their reward for disobeying God and deciding for themselves what's right and wrong. He told them they "would not die" if they did that. But of course they did; and because they did what they did, we die too. Spiritually, in fact, we're born dead. But the real way to become gods is to die in Christ and be reborn in the Spirit, in subjection to God and to the whole Christ, i.e., the risen Jesus and the Church with which he is one body. As an obscure Orthodox saint said: "If you die before you die, you will not die when you die." That's the law of the spirit as opposed to the flesh. Obeying it is true freedom.