Why Do We Do the Things We Do?

by Paul David Tripp

It was a dramatic, game-changing moment. It’s recorded for us in Matthew 5:27–30. Jesus is unfolding the gospel principles of his kingdom. I’ve often wondered about the reaction of the crowd as he spoke these words:

You have heard that it was said, “You shall not commit adultery.” But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart. If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell.

In these words not only does Christ lay out the original intent of God’s law and define where the real moral battle is raging, but he also drops a bomb on any hope that legalism can produce righteous living. Let me give you the helicopter view of this little passage and then draw out some of its implications for the topic of sex.

These words are humbling to hear but vitally important to consider because Christ is saying something counterintuitive to the way most of us think about ourselves and try to make sense out of our lives. From fifty thousand feet, what Christ is doing here in the area of sex is answering the question that every human being asks at some time: Why do people do the things they do? And related to that, why do we say the things we say and make the decisions we make? Why do we successfully fight some things and willfully give in to others? Why do we tell ourselves that we won’t do certain things but end up doing them anyway? Why?

The war of sex is never just a battle with the temptations of the surrounding culture; it is never just about behavior or about what we do with our bodies. Christ is saying that our behavior is directed more by what’s inside us than the people and situations outside us. He’s saying that sexual struggles are inescapably struggles of the heart. Physical adultery is simply the body going where the heart has gone long ago. And as Christ says this, he gives thoughts and desires the moral value of actions. You don’t cross the adultery boundary only when you have illicit sex. You cross the boundary when you give your heart to thoughts and desires that are outside God’s will for you. You will never win the battle with sexual sin by just attempting to harness your behavior, because every wrong sexual act is connected to a decision, which is connected to a desire in your heart. You always give your heart away before you surrender your body to what is wrong.

Here’s where the words of Christ drive us: our struggle with sexual sin is not first a struggle with the environment in which we live or with the people that we live near. Our struggle with sexual sin reveals the dark and needy condition of our hearts. We are our biggest problem. When it comes to sexual sin, the greatest sexual danger to any human being anywhere lives inside him, not outside. Isolation, changes of location and relationship, and management of behavior never work because they don’t target the place where the problem exists—the heart. Sexual struggles have a much deeper beginning point than your eyes and your sexual organs.

So if sexual problems arise from the heart, it’s important to make some biblical observations about the heart. I’m persuaded that you can’t have a real, life-changing conversation about sexual insanity without these heart principles from Scripture.

1. You need to know what the Bible is talking about when it talks about the heart.

Scripture presents the heart as the seat of our emotion, motivation, will, thought, and desire. This means that when you encounter the word heart in your Bible, you should have the following definition in your brain: the heart is the causal center of your personhood. People do what they do because of what’s in their heart. Situations don’t cause you to do what you do. People don’t cause you to do what you do. Locations don’t cause you to do what you do. Your heart does. That’s the Bible’s humbling bottom line.

2. You need to understand that the heart is always functioning under the rule of something.

The heart is a control center. Your heart is always submitting to the rulership of something. And there are only two possibilities. Your heart functions under the control of the Creator or the creation. The problem is not that your heart has the capacity to desire; the problem is ruling desire. The desire for even a good thing becomes a bad thing when that desire becomes a ruling thing. When the pleasures of sex exercise more control over your heart than the will of God does, your heart has already stepped beyond God’s boundaries, and your body will soon follow.

3. You need to realize that what controls your heart will direct your behavior.

Your behavior is inextricably connected to the thoughts and desires of your heart. People and situations may be the occasion and location of what you do, but never the cause. So when you have done with sex what God says you should not do, you can’t look outside yourself for explanations. You must look inside. If, as Jesus says, you’ve already committed adultery in your heart, it won’t be long before you commit the act with the members of your body.

4. You need to realize that this side of eternity, your heart is susceptible.

Because our heart leaves us vulnerable, we must humbly admit that we live in a constant state of susceptibility. None of us has a pure heart. You read it right—not one of us. Yes, by the grace of the cross, the power of sin has been broken, but that doesn’t mean we are sin-free. No, sin still lives with deceptive and destructive power in each one of our hearts, even though its hold is being progressively eradicated by God’s sanctifying grace.

5. You need to admit that this side of eternity, your heart is fickle.

I think we seriously underestimate the fickle nature of our sinful hearts. We quickly switch loyalties. We rapidly trade affection for one thing for another. We all too easily give way to our love. Our hearts will only ever be truly loyal and stable when our hearts are sin-free. As long as sin lives inside us in some way, we’re all sadly shopping for a better, more satisfying master, denying the glory of the Master whom by grace we’ve been given.

6. You need to face the fact that this side of eternity, your heart is deceptive.

We would all like to think that no one knows our heart better than we do. We would like to believe that others may be self-deceived, but we are not. It’s simply not true. Since sin is in its essence deceptive, as long as sin lives in our hearts, we will tend to be blind to the true condition of our hearts. Each person is a participant in the deception of his or her own heart. Since the heart is deceptive, we are often in sexual danger long before our eyes see it and our heart admits it.

7.You need to face the fact that your body will wander where your heart has already gone.

Sexual problems are symptomatic of deeper problems of the heart, and if you give away your heart, you simply will not be successful in controlling your body.

8. You need to confess that your behavior always reveals more about you than about your situation, location, or relationships.

It is here that the evangelical church has tended to lack honesty, integrity, and biblical accuracy. When it comes to the growing sexual insanity that exists in our churches (Internet pornography, marital adultery, and singles having sex), we have tended to point our fingers in the wrong direction. We have talked much about the shocking sexual degradation and coarseness of the surrounding culture. And it is shocking. We talk about the sexual images from which it is almost impossible to protect our children. We point to the sexualization of the fashion and entertainment industries. And we should talk about those things. All these things are issues and need discussion and action, but self-delusion and self-righteousness make the conversation hard and set us up for greater difficulty.

We must all face the fact that changes in our personal sex life don’t begin with cultural analysis; they begin with personal confession. Change doesn’t begin with pointing to the difficulty of your situation or to the behavior of the people around you. Change begins in one place: with heart-deep confession. When it comes to sex, we all need to say that the biggest problem in our sexual lives is us.

Our only hope for personal purity and for a defense against cultural insanity is found in the transformation of our hearts, and for that we need the very same mercy for which David cries out in the beautiful, heart-wrenching prayer of Psalm 51.

Content taken from Sex in a Broken World: How Christ Redeems What Sin Distorts by Paul David Tripp, ©2018. Used by permission of Crossway, a publishing ministry of Good News Publishers, Wheaton, Il 60187,

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