This is a follow up to my sermon from last Sunday.
Christians are not Gnostics. Some Christians have had gnostic tendencies over the course of church history, but Gosticism is an enemy of the faith. Gnostics tend to confuse the physical and the ethical, and thus they find the source of sin in the creation itself. Physical things are tainted, so the goal is to escape or transcend the physical and become completely “spiritual.”
The problem with this view is found in the opening pages of the Bible (actually, the problem is identified on every page of the Bible, but it's best to start at the beginning). God made the world good; indeed, after making man, male and female, as embodied creatures bearing his image, he declares creation “very good.” Physicality and embodiment are not the problem. Before the fall into sin, Adam and the woman would have communed with God through the very physical Tree of Life. That was their sacramental meal. God would meet with them in a physical garden. They were expected to have be fruitful and multiply, which means physical relations between them. They were naked, but there was no shame involved. They we’re commissioned to  have dominion over the earth, which would have meant physical labor and the physical transformation of the creation God made; the raw materials God embedded into the world were to be used in making a God-glorifying civilization. All of this had God’s blessing. That’s the situation in the pristine world of Genesis 1-2.
The fall into sin in Genesis changes things. Adam and Eve lose their communion meal with God at the Tree of Life. The fall means their relationship with God will be “spiritualized,” which must be inferior to the physical, tanglble means of communing with God available to them before they were expelled out of the garden. Of course, over time, God restores physical means of communion with himself, first in Israel’s sacrificial system, and then in the sacraments of baptism and the Lord’s Supper Jesus gives to his church. Sex and work, multiplication and dominion, are still good in themselves in a fallen world, but now they can be used in evil ways. Instead of sex as loving fellowship between a husband and wife, to create new life, sex can be used in selfish and even violent ways to destroy human fellowship. Instead of working to turn the Garden of Eden into the New Jerusalem, now man can use his powerful labor in the world to build cruel and idolatrous anti-kingdoms, like Babel. Physicality can be used for evil ends in a fallen world.
But it is clear from the beginning that God intended for man to use his physicality in righteous ways and this is still God’s purpose. Serving God faithfully can never be a matter of merely having the right thoughts or holding to the right ideas about God. From the beginning, man is called upon to serve God in the creation, in physical ways. Worship in rhe Bible is never merely cerebral; it always invovles bodily acts and physical instruments (today that includes the paper and ink of the Bible, water for baptism, bread and wine for the Lord’s Supper). In a fallen world, our bodies can be used in evil ways, but this is contrary to God’s original design. The whole point of God’s redemptive work is to restore creation to its original design, including the body. Some think that because so much of physical life reveals man’s fleetness that physicality must be the problem. But it is not. The Gnostic tendency is to turn the Christian faith, which is always embodied in various ways in Scripture, into mere ideology, a disembodied thing. Again, this is a mistake of monstrous proportions.
C. S. Lewis pointed out the raw physicality of the Christian faith by saying, “Of course God likes matter; he invented it.” Not only that but the eternal Son himself took a body in the incarnation, and entered into a glorified body forever in the resurrection. There is no way to tell the gospel story without getting enmeshed in the physical world. 
All this to say: In Mark 9:43ff, when Jesus talked about cutting off body parts if they cause us to sin, we need to set that within the context of the whole biblical narrative. The body as such is never blamed for sin in Scripture. The body can be an instrument of sin — but it can also be an instrument of righteousness. And that’s the whole point of the passage. The body is a battlefield, but who wins and loses on the battlefield of the body is not determined by the body itself but by the heart that directs the body.
Scripture views man as an integrated whole with a physical and non-physical/psychical aspect. Man is both body and soul. Body and soul together comprise the image of God in man. The eternal Son took to himself a human nature, meaning both body and soul, in the incarnation. Our redemption involves both body and soul. It is not enough to have a new heart or a new body; we need both to be fully saved.
As I said in the sermon, there is something cryptic about Jesus’ words.’ If your body causes you to sin — but that’s a big “if." Your body, or the various members of your body, are clearly not the actual source of your sin. Sin must be traced back to its heart, which is the human heart, from which evil desires spring forth, which them come to fruition in and through the body. Man is completely integrated; it really makes no sense to try to distinguish bodily sins from soul sins. But sin cannot be eradicated by lopping off offending body parts that get implicated in sinful actions. To get at the heart of human sin, we have to deal with the human heart.
We tend to think the body defiles the soul, but if anything it is the other way around. The heart uses the body in sinful ways. Of course, the body is all too willing to partner with the heart in rebelling against God’s order and design. Sin feels good. Sin appeals to the body’s senses, it is sensual, it is pleasurable. Many temptations come to us in bodily ways. 
Redemption redirects how we use our bodies because when God redeems us he reorients the desires of our hearts. We begin to offer our bodies, and the members of our bodies, to righteousness. We begin to use our bodies as they were designed to be used. This is difficult precisely because the residue of the fall — not coincidentally called the “flesh” in the Scriptures — still clings to us. 
All of this means exactly what I indicated in the sermon: The Christian life is a war. It is a fight to the death. Sin must be killed or sin will kill us. By the power of God’s Spirit, our hearts and bodies can be directed towards righteousness, but it does not come naturally or easily even after we have been living the Christian faith or a long time. There is a cost. The battle will often leave us battered, bruised, and scarred, metaphorically at least as we engage in this agonizing contest with sin. It takes persistence, discipline, and love to win the war with sin. The Christian life is cruciform — it is cross-shaped. And if the Christian life is the life of the cross — the life of becoming a living sacrifice — it will surely be hard and painful because death always hurts. But the good news is that all who are willing to fight sin in the name of Jesus and the power of the Holy Spirit are assured glory and victory. Indeed, the victory is the glory in this case. The reward for fighting sin in the body is a new body, a resurrection body, at the last day. And that resurrection body means glory, inside and out. At the last day, we will see death completely vanquished as we enter into eternal life.
Too many professing Christians today are spiritual pacifists. They have not taken up the weapons Gd has given us to fight against their own sin. They are embarrassed to think of the Christian life as a battleground, or their bodies as a battlefield. Several denominations, such as liberal Presbyterians and Methodists have dropped hymns like “Onward Christian Soldiers” because they want a pacifist, peaceful Christian life. But so long as we still sin, there can be no peace. The war rages on — and if we are not willing to fight it, we will lose it, and if we lose it, we lose our souls forever.
Winning the war on sin does not mean you have make a certain degree of progress on the battlefield in order to be saved. The key is not the perfection of your life, but the direction of your life. If you are fighting sin at all — really fighting sin (and not merely sin's consequences/punishments)  — then you are a true Christian, assured of ultimate and final victory. But many who claim Christ do not take up their cross, do not fight against sin, and are dangerously deceived. Do not let that be you. Fight the good fight.
Recommended reading: Holiness by J. C. Ryle (especially chapter 4)