Spiritual Cancer: A Sermon for the Eighth Sunday after Trinity on St. Matthew 7:15–23

Wolf in Sheep's Clothing

Sermon audio here.

Some things are obviously dangerous. You know not to put your hand in a fire because it’s really hot and you’ll get burned. Eating a gallon of ice cream, on the other hand, tastes delicious—but it’s not good for you. If you came up with a list of all the dangerous things you could think of, ice cream probably wouldn’t be on it. How many years did people smoke before they found out it causes cancer? The danger of some things isn’t immediately apparent.

High blood pressure is often referred to as the silent killer because it increases the risk of heart disease and stroke—the first and third most common causes of death in the United States. High blood pressure often has no warning signs or symptoms. Hypertension can be deadly, but unless you’re checking for it, you might not even know you have it.

But even deadlier than high blood pressure—and even harder to detect—is false doctrine. The danger of false doctrine isn’t immediately apparent. It’s not like fire, which burns you if you get too close. But just because it doesn’t physically hurt you doesn’t make it safe.

The devil tries to convince us that the most dangerous things in the world are things that cause physical pain—sticks and stones. But false doctrine is much deadlier than cancer. The worst cancer can do is rot your body; false doctrine can rot your soul. As important as it is to guard your health, it’s even more important to guard your doctrine.

False doctrine is sort of like eating a gallon of ice cream: it seemed like a good at the time, but can have serious consequences later. False doctrine even sounds good to sinful human ears. Satan didn’t have to work very hard to entice Eve to believe his lies. After listening to Satan, our First Parents thought sinning seemed “good”, “delight[ful]”, and even “desirable” (Genesis 3:6). If anything deserves to be called the silent killer, it’s false doctrine—the leading cause of spiritual cancer.

False doctrine is nothing other than the devil’s lies masquerading as the truth. Now we know unbelief is the greatest sin there is, and that’s something we usually take pretty seriously. But we don’t get as worked up about false doctrine, even though it’s poisonous to faith. Satan’s done a fine job of getting us to forget that the bitter fruit of false doctrine is unbelief.

Getting upset about someone not believing in God but not caring about false doctrine is sort of like getting upset about losing your house but not caring that it’s on fire and burning to the ground. Unbelief stems from the false doctrine that there’s no God, or that He’s evil or powerless, or that we aren’t sinful, or any other of the devil’s lies. If you care about faith, you must care about doctrine.

The greatest sins aren’t disobedience, murder, adultery, theft, false testimony or coveting. The greatest sins are theological—believing and teaching false doctrine. God’s Name is hallowed when the Word of God is taught in its truth and purity, and we, as the children of God, also lead holy lives according to it (Small Catechism, first petition).

I was talking about this with Joshua the other day in terms of Legos. You have to pay close attention when you’re building a complicated set and follow the directions carefully. It’s easy to make a small mistake and not notice it at the time, but if you don’t catch it, the whole thing’ll be ruined. As carefully as you have to follow the directions when building Lego sets, it’s even more important to pay careful attention to doctrine. A wrong Lego can ruin a set, but wrong doctrine can ruin your faith.

Not even Solomon, for all his wisdom, took false doctrine as seriously as he should have. God told him not to marry non-Christians because their false theology would eventually rub off on him. But Solomon didn’t listen. He figured he was secure in his faith—which, by the way, is another one of the devil’s favorite temptations. Scripture says take heed lest you fall (1 Cor. 10:12). Satan says, “You’re a good Christian. You’ll be fine, go do whatever you like.” Believe it or not, Satan wants you to think you’re a good Christian. God wants you to repent.

Doctrine doesn’t matter to the secure. They vainly imagine their faith can weather any storm. Solomon took his faith for granted, and sure enough, his wives led him astray.

That’s why, like high blood pressure, you have to constantly guard against false doctrine. The most precious gift you have in this life is faith in Christ, and there’s nothing more deadly to faith than false doctrine. The difference between pure & false doctrine is the difference between truth & error, heaven and hell, Christ and Satan.

In order to guard against false doctrine, you have to be able to spot it. Those who teach false doctrine, our Lord says, don’t look threatening. It’s not like they show up with ski masks and weapons and demand all your catechisms. They’re wolves in sheep’s clothing. Sometimes they put on vestments and chant. Satan will do everything he can to look the part. He even disguises himself as an angel of light.

Every Christian has the vocation of theologian. Jesus wants you to know Scripture so well you could practically teach at the seminary. Studying the bible isn’t just for pastors. How can you guard against false doctrine if you don’t know what Scripture teaches? Pastors can err. The devil even quotes the bible to suit his purposes, as he did when he tempted our Lord (Matthew 4:1–11).

Spending time each day in God’s Word is essential to guarding against spiritual cancer. Biblical ignorance puts you at risk of being tossed to and fro by the waves and carried about by every wind of doctrine (Ephesians 4:14).

Those who delight in the Word of God and meditate on it day and night are like a massive tree that isn’t shaken or easily uprooted. They bear good fruit and prosper in all things (Psalm 1:2–3). A healthy faith gives you peace and joy at all times, especially when life seems to be giving you nothing but rotten fruit.

As the Great Physician, our Lord shows us the warning signs of those who suffer from false doctrine. He diagnoses two trees, one healthy and the other diseased. Nurtured by pure doctrine, the healthy tree produces the good fruits of faith (cf. Psalm 1:2–3).

The bad tree is the one that has become infected by false doctrine. Our Lord says the only thing that tree’s good for is to be cut down and thrown into the fire (Matthew 7:19). Not only does false doctrine threaten to rob you of eternal life, it bears nothing but rotten fruit now. There is no love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, or self-control in those who suffer from spiritual cancer. False doctrine bears nothing but the rotten fruits of anger, misery, discontentment, impatience, harshness, depravity, and intemperance.

Good physicians do everything they can to keep you well, but sometimes they fail. Jesus is no good physician, He’s the Great Physician, and He never fails. He is the remedy for your spiritual cancer. He bore all of your sin in His body and died for them on the cross—even the sins you don’t see and of which you were never aware. His baptismal waters cover them all. He who neither slumbers nor sleeps guards your life at all times and promises that not even the most cleverly disguised wolf can snatch you out of His hand.

Satan may be good at deception, but not even he could out-deceive our Lord. Jesus needed no disguise. He gave Satan exactly what he wanted: His dead corpse hanging on a cross. Death is the last thing Satan would’ve ever expected God to use to cure spiritual cancer. Who would have thought God would use the wages of sin to destroy sin? Satan certainly didn’t.

Jesus overcame Satan by throwing his false doctrine right back into his face. For the devil, the death of Jesus was a dream come true. He actually thought the power of death was greater than the power of God. Even though God once brought all creation into being out of nothing, still Satan doubted God’s ability to bring life where there once was nothing but a dead corpse.

Christ is risen, dearly beloved, and this means the only one who’s terminal is Satan and those who follow him in his rejection of Christ. The risen Lord comes to you today to bring His healing Gospel, to absolve away your spiritual cancer, and to feed you with His incorruptible Body and Blood. Spiritual cancer cannot abide in the presence of the Lord of Life.

Only don’t limit your access to this Healing Word to once per week. Jesus doesn’t want you to have a pastor only on Sunday morning; He wants you to have a pastor at all times. Call or email me anytime, whether you’re sick, in the hospital, want confession, have a question, or need pastoral care of any kind.

Jesus also gives you Holy Scripture that He might come to you daily. Hear His tender invitation from Psalm 78:

Give ear, O my people, to my | teaching;*
incline your ears to the words | of my mouth! (v. 1)

There’s nothing better for your faith than to give ear to the Lord’s teaching. Your flesh and heart may fail, but God is the strength of your heart and your portion forever (Psalm 73:26).

Soli Deo Gloria

+Rev. Eric Andersen
St. Matthew 7:15–23
The Eighth Sunday after Trinity, 2015: “Spiritual Cancer”
Zion, Summit: https://www.facebook.com/zionlcms
Immanuel, Hodgkins: https://www.facebook.com/immanuelhodgkins
Around the Word Bible Studies: http://www.whatdoesthismean.org/bible-studies.html
Steadfast Throwdown: http://steadfastthrowdown.org/



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