Carved in the bark of every tree in the garden of God are the words “If it dies, it bears much fruit” (John 12:24). Three words are branded into the flesh of every Christian: “You have died” (Colossians 3:3). And the heartfelt confession of every believer is “I have been crucified with Christ” (Galatians 2:20).

But what does this mean? Who died when I became a Christian? Answer: my flesh died. “Those who belong to Christ Jesus have crucified the flesh” (Galatians 5:24).

But what does flesh mean? Not my skin. Not my body. That can be an instrument of righteousness (Romans 6:13). The “works of the flesh” are things like idolatry and strife, anger and envy (Galatians 5:19–21) — attitudes, not just immoral acts of the body.

How the Bible Defines Flesh

The closest thing to a biblical definition of the flesh is Romans 8:7–8: “The mind that is set on the flesh is hostile to God, for it does not submit to God’s law; indeed it cannot. Those who are in the flesh cannot please God.”

So the flesh is the old “me” who used to rebel against God. In the flesh I was hostile and insubordinate. I hated the thought of admitting I was sick with sin. I defied the idea that my greatest need was a Good Physician to make me well. In the flesh I trusted my wisdom, not God’s. So nothing I did in the flesh could please God, because “without faith it is impossible to please God” (Hebrews 11:6). And the flesh does nothing from faith.

So the flesh is the old self-reliant, faithless me. This is what died when God saved me. God clamped the arteries on my old unbelieving heart of stone. And when it died, he took it out and gave me a new heart (Ezekiel 36:26).

What’s the difference between this new heart that lives and the old one that died? The answer is given in Galatians 2:20. It says, “I have been crucified with Christ. . . . And the life I now live, I live by faith in the Son of God, who loved me and gave himself for me.” The old heart that died trusted in itself; the new heart banks on Christ every day.

Fight Sin By Trusting Jesus

This is the answer to our first question: How do dead people do battle with sin? They do battle with sin by trusting the Son of God. They are dead to Satan’s lie, which goes like this: “You will be happier if you trust your own ideas about how to be happy instead of trusting the counsel and the promises of Christ.” Christians have died to that deceit. So the way they fight Satan is by trusting that the paths and promises of Christ are better than Satan’s.

This way of doing battle with sin is called the “fight of faith” (1 Timothy 6:12; 2 Timothy 4:7). The victories of this fight are called the “works of faith” (1 Thessalonians 1:3; 2 Thessalonians 1:11). And in this warfare Christians become holy by faith (Acts 26:18; 2 Thessalonians 2:13).

Let’s think, then, about this fight of faith. It is not like war games with rubber bullets. Eternity is at stake. Romans 8:13 is a key verse: “If you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live.” This is written to professing Christians, and the point is that our eternal life hangs on our battle with sin.

It does not mean that we earn eternal life by killing sin. No, it is “by the Spirit” that we fight. He will get the glory, not us. Nor does Romans 8:13 mean that we fight with an anxious sense of uncertainty about winning. On the contrary, even as we fight we have confidence that “he who began a good work in [us] will complete it at the day of Jesus Christ” (Philippians 1:6). Nor does Romans 8:13 mean that we must be perfect now in our victory over sin. Paul renounces the claim to perfection (Philippians 3:12).

What God Requires

The demand in Romans 8:13 is not sinlessness but mortal combat with sin. This is utterly essential in the Christian life. Otherwise, we give no evidence that the flesh has been crucified. And if the flesh has not been crucified, we do not belong to Christ (Galatians 5:24). The stakes in this battle are very high. We are not playing war games. The outcome is heaven or hell.

How then do dead people “put to death the [sinful] deeds of the body”? We have answered, “by faith!” But just what does this mean? How do you fight sin with faith?

Suppose I am tempted to lust. Some sexual image pops into my brain and beckons me to pursue it. The way this temptation gets its power is by persuading me to believe that I will be happier if I follow it. The power of all temptation is the prospect that it will make me happier. No one sins out of a sense of duty when what they really want is to do right.

So what should I do? Some people would say, “Remember God’s command to be holy (1 Peter 1:16), and exercise your will to obey because he is God!” But something crucial is missing from this advice — namely, faith. A lot of people strive for moral improvement who cannot say, “The life I live I live by faith” (Galatians 2:20). A lot of people try to love who don’t realize that “what counts is faith working through love” (Galatians 5:6).

The fight against lust (or greed or fear or any other temptation) is a fight of faith. Otherwise the result is legalism. I’ll try to explain how we fight sin with faith.

Fighting Sin By the Spirit

When the temptation to lust comes, Romans 8:13 says, “If you kill it by the Spirit, you will live.” By the Spirit! What does that mean? Out of all the armor God gives us to fight Satan, only one piece is used for killing — the sword. It is called the sword of the Spirit (Ephesians 6:17). So when Paul says, “Kill sin by the Spirit,” I take that to mean, “Depend on the Spirit,” especially his sword.

What is the sword of the Spirit? It’s the word of God (Ephesians 6:17). Here’s where faith comes in. “Faith comes from hearing, and hearing through the word of Christ” (Romans 10:17). The word of God cuts through the fog of Satan’s lies and shows me where true and lasting happiness is to be found. And so the word helps me stop trusting in the potential of sin to make me happy, and instead entices me to trust in God’s promise of joy (Psalm 16:11).

I wonder how many believers today realize that faith is not merely believing that Christ died for our sins. Faith is also being confident that His way is better than sin. His will is more wise. His help is more sure. His promises more precious. And his reward more satisfying. Faith begins with a backward look at the cross, but it lives with a forward look at the promises. Abraham “grew strong in his faith . . . fully convinced that God was able to do what he had promised” (Romans 4:20–21). “Faith is the assurance of things hoped for” (Hebrews 11:1).

When faith has the upper hand in my heart I am satisfied with Christ and his promises. This is what Jesus meant when he said, “He who believes in me shall never thirst” (John 6:35). If my thirst for joy and meaning and passion are satisfied by the presence and promises of Christ, the power of sin is broken. We do not yield to the offer of sandwich meat when we can see the steak sizzling on the grill.

Satisfaction Slays Sin

The fight of faith is the fight to stay satisfied with God. “By faith Moses . . . [chose] rather to be mistreated with the people of God than to enjoy the fleeting pleasures of sin, . . . for he was looking to the reward” (Hebrews 11:24–26). Faith is not content with “fleeting pleasures.” It is ravenous for joy. And the word of God says, “In God’s presence is fullness of joy, and in his right hand are pleasures forevermore” (Psalm 16:11). So faith will not be sidetracked into sin. It will not give up so easily in its quest for maximum joy.

The role of God’s word is to feed faith’s appetite for God. And in doing this it weans my heart away from the deceptive taste of lust. At first lust begins to trick me into feeling that I would really miss out on some great satisfaction if I followed the path of purity. But then I take up the sword of the Spirit and begin to fight. I read that it is better to gouge out my eye than to lust (Matthew 5:29). I read that if I think about things that are pure and lovely and excellent the peace of God will be with me (Philippians 4:8–9). I read that setting the mind on the flesh brings death, but setting the mind on the Spirit brings life and peace (Romans 8:6).

And as I pray for my faith to be satisfied with God's life and peace, the sword of the Spirit carves the sugar coating off the poison of lust. I see it for what it is. And by the grace of God, its alluring power is broken.

Forward-Looking Faith

This is the way dead people do battle with sin. This is what it means to be a Christian. We are dead in the sense that the old unbelieving self (the flesh) has died. In its place there is a new creation. What makes it new is faith. Not just a backward-looking belief in the death of Jesus, but a forward-looking belief in the promises of Jesus. Not just being sure of what he did do, but also being satisfied with what he will do.

With all eternity hanging in the balance, we fight the fight of faith. Our chief enemy is the lie that says sin will make our future happier. Our chief weapon is the Truth that says God will make our future happier. And faith is the victory that overcomes the lie, because faith is satisfied with God.

The challenge before us then is not merely to do what God says because He is God, but to desire what God says because he is good. The challenge is not merely to pursue righteousness, but to prefer righteousness. The challenge is to get up in the morning and prayerfully meditate on the Scriptures until we experience joy and peace in believing “the precious and very great promises” of God (Romans 15:13; 2 Peter 1:4). With this joy set before us the commandments of God will not be burdensome (1 John 5:3) and the compensation of sin will appear too brief and too shallow to lure us.