Sin can grow.
...like a vine that starts small but smothers an entire wall—sin can grow.
...like a cute cub that becomes a lion—sin can grow.
...like a tapeworm that starts microscopic but can ruin a body—sin can grow.
"Each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death" (James 1:14-15).
No wonder we're commanded to "kill" sin (Romans 8:13). This is no joking matter. It's not 'what we as good Christians should do.' It's dangerous, deadly, damning.
It's sin that infects. It sin that brings sorrow. It's sin that sends people to hell. It's sin that spreads, taking over lives in various ways. It's sin that leads to everlasting "death" (James 1:15; Romans 6:23).
What It Could Become
But we're often hesitant to kill it until we see a certain sin as noticeably terrible. To us there's no need to avoid that situation, not watch that movie, or not go to that place until it becomes bad enough. But, as in all the analogies above, it isn't what sin presently is that's dangerous, per se, it's what it could become.
It's better to cut the vine before it takes over the wall.
It's wiser to not live with the cub at all before it becomes a lion.
It Can Transform
And if that wasn't enough, sin not only can grow, it can transform. Like a maggot that turns into a fly, we might think we have it under control and so leave it. But then, next thing we know it, it's brought on us other temptations, other desires, other sins.
By letting it live, we've lost control, all because we thought it wasn't harmful.
Sin, Grace, and the Gospel
We’d do well, then, to feel the dreadfulness in sin. The gospel is true and helpful here: The good news includes the truth that no amount of sin can overcome grace (Romans 5:20) and the truth that sin is dangerous and deadly, even to professing believers. Both are gospel-facts, and we need both.
It seems to me that in modern evangelicalism—and in my own life—we often emphasize grace in a way where we forget the deadliness of sin. We think that since we're Christians, sin has lost its dangerousness. But the New Testament writers didn't think so. To them, sin in the Christian's heart must be taken seriously. Remember, both James and Paul, quoted above, were writing to Christians. We should heed their warnings: Christian, "desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death" (James 1:15). Christian, "the wages of sin is death" (Romans 6:23). Or as the author of Hebrews put it (again, to Christians): Christian, "strive for the holiness without which no one will see the Lord" (Hebrews 12:14).
This also impacts us positively: it fuels our gospel-joy. By seeing the danger of sin, we gain greater appreciation for the Sin-Sacrifice, the Sin-Taker, the Sin-Destroyer, Jesus Christ. It illuminates the glory of the gospel: "For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God" (2 Corinthians 5:21). It gives stunning weight to the fact that "he does not deal with us according to our sins, nor repay us according to our iniquities...as far as the east is from the west, so far does he remove our transgressions from us" (Psalm 103:10, 12). It allows us to more robustly praise "him who loves us and has freed us from our sins by his blood" (Revelation 1:5).
In the gospel, we both see the dreadfulness of sin and so "put to death the deeds of the body" (Romans 8:13), and we can cry with joy, "Blessed is the one whose transgression is forgiven, whose sin is covered" (Psalm 32:1). We fight sin, knowing its dangerous, but we do so by faith in that it's been defeated. We terminate sin, knowing its deadly, but we do so trusting that the Sin-Taker died in our place.
Sin can grow, so let's realize it's deadliness and kill it. And also, let's be thankful: by grace, it's been killed and covered. Praise be to Christ.