"I've repented of my sin, put my faith in Jesus Christ alone for my salvation, but there's still sin in my life. How hard am I supposed to fight against it? It's not going to damn me to hell, so what's the big deal?"
Good question. If we are secure in our salvation, to what extent do we battle against sin? A couple of things can be said:
1) Jesus was very serious about Christians battling hard against sin.
Matthew 5:29-30 — "If your right eye causes you to stumble, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. And if your right hand causes you to stumble, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to go into hell."
How would you like your right eye to be plucked out? With a spoon? Knife? Just a few bony fingers? Ouch! Eyelashes in the eye are painful enough...I can't imagine this!
Now of course, we aren't meant to take this literally. If we were, everybody would be walking around without right eyes and hands. However, the figurative imagery is meant to convey extremes! In other words, Jesus wants us to go to great lengths to battle against sin! If we weren't meant to fight hard against sin, he wouldn't have used such an extreme word picture.
I once heard a pastor give some tactics to people battling against pornography. One of the several suggestions he listed was to read and study Wayne Grudem's Systematic Theology in order to deepen one's view of God. That book is over 1200 pages long! Unreasonable? Not at all!
2) Heaven will be a bummer to unholy people.
Can we all agree heaven will be a place of perfect holiness? If holiness is not a joy to you now, why would it be a joy to you in heaven? Of course, the unrighteous will not inherit the kingdom of God (1 Cor. 6:9; Gal. 5:21). That's the point. Charles Spurgeon put it well, "Sooner could a fish live upon a tree than the wicked in Paradise." To fight hard against sin is to long for holiness. To long for holiness is to long for God and heaven itself.